|
|
Line 1,175: |
Line 1,175: |
| this question further, though I would fain express my conviction that had the observations recorded in this paper been | | this question further, though I would fain express my conviction that had the observations recorded in this paper been |
| earlier available, much vain speculation as to the phytogeny | | earlier available, much vain speculation as to the phytogeny |
| of the trophoblast might possibly have been avoided. | | of the trophoblast might possibly have been avoided. |
| | |
| ==Chapter VII. - The Early Ontogeny of the Mammalia in the Light of the Foregoing Observations==
| |
| | |
| In entering on a discussion of the bearings of the results
| |
| of my study of the early development of Marsupials on
| |
| current interpretations of early Mammalian ontogeny, and
| |
| especially of the homologies of the germ-layers, I desire at
| |
| the outset to emphasise my conviction that, specialised
| |
| though the Marsupials undoubtedly are in certain features of
| |
| their anatomy, e. g. their dentition, genital ducts, and mammary apparatus, the observations recorded in the preceding
| |
| pages of this paper afford not the slightest ground for the
| |
| supposition that their early ontogeny is also of an aberrant
| |
| type, devoid of signiffcance from the point of view of that of
| |
| other mammals. On the contrary, I hope to demonstrate
| |
| that the Marsupial type of early development not only readily falls into line with that of Eutheria, and with what we know
| |
| of the early development of the Prototheria, but furnishes
| |
| ns with the key to the correct interpretation of that extraordinarily specialised developmental stage, the Eutherian
| |
| blastocyst. In particular I hope to show that the description
| |
| which I have been able to give of the mode of formation of
| |
| the' Marsupial blastocyst, bridges in the most satisfactory
| |
| fashion the great gap which has till now existed in our
| |
| knowledge of the way in which the transition from the
| |
| Monotrematous to the Eutherian type of development has
| |
| been effected.
| |
| | |
| 1. The Early Development of the Monotremata.
| |
| | |
| Our knowledge of the early development of the oviparous
| |
| mammals is admittedly still far from complete. Nevertheless
| |
| it is not so absolutely fragmentary that it can be passed over
| |
| in any general discussion of early mammalian ontogeny, and
| |
| I certainly cannot agree with the opinion of Assheton ('08,
| |
| p. 227) that from it “we gain very little help towards the
| |
| elucidation of Eutherian development.†On the contrary, I
| |
| think that the combined observations of Semon ('94), and
| |
| Wilson and Hill ('07) shed most valuable light on the early
| |
| ontogenetic phenomena in both the Metatheria and Eutheria.
| |
| I propose therefore to give here a very brief resume of the
| |
| chief results of these observers,^ and at the same time to
| |
| indicate how the knowledge of early Monotreme ontogeny
| |
| we possess, limited though it be, does help us to a better
| |
| understanding of the phenomena to which I have just
| |
| referred.
| |
| | |
| The ovum, as is well known from the observations of
| |
| Caldwell. ('87), is Reptilian in its character in all but size.
| |
| It is yolk-laden and telolecithal, the yolk consisting of
| |
| discrete yolk-spheres, and it is enclosed outside the zona
| |
| (vitelline membrane) by a layer of albumen and a definite shell.
| |
| | |
| * In so doinff I have largely utilised the phraseology of Wilson and
| |
| Hill's paper ('07).
| |
| | |
| | |
| THE EARLY DEVELOPMENT OP THE MARSUPIALIA. 87
| |
| | |
| | |
| At the moment of entering the oviduct it has a diameter of
| |
| 3‘5-4 mm. (2‘5-3 mm. according to Caldwell), and is therefore
| |
| small relatively to that of a reptile of the same size as the
| |
| adult Monotreme, but large relatively to those of other
| |
| mammals, being about twelve times larger than that of
| |
| Dasyurus, and about eighteen times larger than that of the
| |
| rabbit.
| |
| | |
| Cleavage is meroblastic. The first two cleavage planes are
| |
| at right angles to each other, as iii the Marsupial, and divide
| |
| the germinal disc into four approximately equal-sized cells
| |
| (Semon, Taf. ix, fig. 30). Each of these then becomes subdivided by a meridional furrow into two, so that an 8-celled
| |
| stage is produced, the blastomeres being arranged symmetrically, or almost symmetrically, on either side of a median line,
| |
| perhaps corresponding to the primary furrow (Wilson and Hill,
| |
| p. 37, text-figs. 1 and 2). Imagine the yolk removed and the
| |
| blastomeres arranged radially, and we have at once the open
| |
| ring-shaped 8-celled stage of Dasyurus. The details of the
| |
| succeeding cleavages are unknown. Semon has described a
| |
| stage of about twenty-four cells (Semon, Taf. ix, fig.31),inwhich
| |
| the latter formed a one-layered circular plate with no evidence
| |
| of bilateral symmetry, and this is succeeded by a stage also
| |
| figured by Semon (figs. 32 and 33, cf. also Wilson and Hill,
| |
| PI. 2, fig. 2), in which the blastoderm has become sevei'al
| |
| cells thick, though it has not yet increased in surface extent.
| |
| It is bi-convex lens-shaped in section, its lower surface being
| |
| sharply limited from the underlying white yolk. No nuclei
| |
| are recognisable in the latter, either in this or any subsequent
| |
| stage, nor is there ever any trace of a syncytial germ-wall,
| |
| features in which the Monotreme egg differs from the
| |
| Sauropsidan.
| |
| | |
| The next available stage, represented by an egg of Ornithorhynchus, described by Wilson and Hill ('07, p. 38, PI. 2, fig.
| |
| 4), and by an egg of Echidna, described by Semon ('94, p. 69,
| |
| figs. 22 and 33), is separated by a considerable gap from the
| |
| preceding, and most unfortunately so, since it belongs to the
| |
| period of commencing formation of the germ-layers. The
| |
| | |
| | |
| 88
| |
| | |
| | |
| J. r. HILL.
| |
| | |
| | |
| cellular lens-shaped blastoderm of the preceding stage has
| |
| now extended in the peripheral direction so as to enclose
| |
| about the upper half of the yolk-mass, and in so doing it has
| |
| assumed the form, almost exclusively, of a unilaminar thin
| |
| cell-membrane, composed of flattened cells and closely applied
| |
| to the inner surface of the zona. At the embryonic pole,
| |
| however, in the region of the white yolk-bed, there are
| |
| present in the Ornithorhynchus egg a few plump cells,
| |
| immediately subjacent to the unilaminar blastoderm, but
| |
| separate and distinct from it, whilst in the Echidna egg
| |
| Semon's figure (fig. 33), which is perhaps somewhat schematic,
| |
| shows a group of scattered cells, similar to those in the
| |
| Ornithorhynchus egg but placed considei'ably deeper in the
| |
| white yolk-bed. Unfortunately we have no definite evidence
| |
| as to the significance of these internally situated cells. One
| |
| of two possible interpretations may be assigned to them.
| |
| Either they represent the last remaining deeply placed cells
| |
| of the blastodisc of the preceding stage, which have not yet
| |
| become intercalated in the unilaminar blastodermic membrane
| |
| believed by Semon to be the condition attained in eggs of
| |
| about this stage of development, or they are cells which have
| |
| been proliferated off from this unilaminar blastoderm, to
| |
| constitute the parent cells of the future yolk-entoderm. As
| |
| regards Echidna, Semon expresses a definite enough opinion ;
| |
| he holds that these deeply placed cells actually arise by a
| |
| somewhat diffuse proliferation or ingrowth from a localised
| |
| depressed area of the blastoderm at the embryonic pole, and
| |
| that they give origin to yolk-entoderm. This interpretation
| |
| of Semon seems probable enough in view of the mode of origin
| |
| of the entoderm in the Metatheria and Eutheria. Moreover
| |
| in the next available stage, an egg of Ornithorhynchus, just
| |
| â– over 6 mm. in diameter, described by Wilson and Hill, the
| |
| blastoderm is already bilaminar throughout its extent, so that
| |
| we .might veiy Avell expect to find the beginnings of the entoderm in the somewhat younger eggs.
| |
| | |
| In the 6 mm. egg just referred to, the peripheral portion of
| |
| the utjilaminar blastoderm of the preceding stage has grown
| |
| | |
| | |
| 89
| |
| | |
| | |
| THE EARLY DEVELOPMENT OF THE MARSUPIALIA.
| |
| | |
| SO as to enclose the entive yolk-mass in a complete ectodeimal
| |
| envelope, whilst iiiteimally to that a complete lining of yolkentoderm has become established. As tlie result of these
| |
| changes, and of the imbibition of fluid from the uterus, the
| |
| solid yolk-laden egg has become converted into a relatively
| |
| thin-walled vesicle or blastocyst, possessed of a bilaminar
| |
| wall surrounding the partly fluid vitelline contents of the egg.
| |
| Throughout the greater part of its extent the structure of the
| |
| vesicle wall is very simple. It consists externally of an
| |
| extremely attenuated ectodermal cell - membrane closely
| |
| adherent to the deep surface of the vitelline membrane
| |
| (zona), and within that of a layer of yolk-entoderm, composed
| |
| of large swollen cells, containing each a vesicular nucleus,
| |
| and a number of yolk-spheres of varying size. Over a small
| |
| area, overlying the white yolk-bed, however, the ectodermal
| |
| layer of the wall presents a different character to that
| |
| described above. Its constituent cells are here not flattened
| |
| and attenuated, but irregnlai'ly cuboidal in form and much
| |
| more closely packed together; moreover they stand in proliferative continuity with a subjacent mass of cells, also in
| |
| process of division. The irregular superficial layer and this
| |
| latter mass together form a thickened lenticular cake, "5 mm.
| |
| in greatest diameter, projecting towards the white yolk-bed
| |
| but separated from it by the yolk-entoderm, which retains
| |
| its character as a continuous cell-membrane. This differentiated, thickened area of the wall, situated as it is at the upper
| |
| pole of the egg, as marked by the white yolk-bed, must be
| |
| held to represent a part of the future embryonal region.
| |
| Wilson and Hill incline to regard it as in some degree the
| |
| equivalent of the “primitive plate†of Eeptiles and as the
| |
| initial stage in the formation of the primitive knot of latex;
| |
| eggs. This question, however, does ixot closely concern us
| |
| here : the point I wish to emphasise is the relative inactivity
| |
| of the cells composing the embryonal region of the blastoderm
| |
| in the Monotreme as compared with the marked activity displayed by those constituting the peripheral (extra-embryonal)
| |
| region of the same. It is these latter cells which by their
| |
| | |
| | |
| 90
| |
| | |
| | |
| J. P. HILL.
| |
| | |
| | |
| rapid growth complete the envelopment of the yolk-mass and
| |
| so constitute the lower hemisphere of the blastocyst.
| |
| | |
| Ihe bilaminar blastocyst of the Monotreme, foi'nied in the
| |
| manner indicated above, is entirely comparable with the
| |
| Marsupial blastocyst of the same developmental stage. There
| |
| are differences in detail certainly (e.g. in the characters,
| |
| time of formation, and rate of spreading of the entoderm,
| |
| in the mode of formation of the blastocyst cavity and in its
| |
| contents, in the apparent absence in the Monotreme of any
| |
| well-marked line of division between the embryonal aud extraembryonal regions of the ectoderm, in the relatively earlier
| |
| appearance of differentiation in the embryonal region in the
| |
| Monotreme as compared with the Marsupial), but the agreements are obvious and fundamental ; in particular, I would
| |
| emphasise the fact that in both the embryonal region is
| |
| superficial and freely exposed, and forms part of the blastocyst wall just as that of the reptile forms part of the general
| |
| blastoderm. Moreover, should future observations confi^rm
| |
| the view of Semon that the primitive entodermal cells of the
| |
| Monotreme are proliferated off from the embryonal region of
| |
| the unilaminar blastoderm, then we should be justified in
| |
| directly comparing the latter with the unilaminar wall of the
| |
| Marsupial blastocyst, and in regarding it also as consisting
| |
| of two differentiated regions, viz. a formative or embryonal
| |
| region, overlying the white yolk-bed, and giving origin to
| |
| the embryonal ectoderm and the yolk-entoderm, and a nonformative region which rapidly overgrows the yolk-mass so
| |
| as to eventually completely enclose it, just as does the less
| |
| rapidly growing extra-embryonal ectoderm of the Sauropsidan blastoderm.^ Meantime I see no reason for doubting
| |
| that this rapidly growing peripheral portion of the unilaminar
| |
| blastoderm of the Monotreme is anything else than extraembryonal ectoderm homogenous with that of the reptile.
| |
| Indeed, I am not aware that any embryologist except Hubrecht
| |
| thinks otherwise. Even Asshetou is, I believe, content to
| |
| | |
| * We should further he justified in concluding that the entoderm is
| |
| similar in its mode of origin in all three mammalian sub-classes.
| |
| | |
| | |
| THE EAHLY DEVELOPMENT OF THE MARSUPIALIA. 91
| |
| | |
| | |
| regard the outer layer of the Monotrerae blastocyst ns
| |
| ectodermal. Hubrecht's view is that the primitive eiitodermal
| |
| cells of Semon give origin, not to yolk-entoderm, but to the
| |
| equivalent of the embryonal knot of Eutheria, whilst the
| |
| uuilaminar blastodermic membrane itself is a larval layer
| |
| - the trophoblast - that portion of it overlying the internally
| |
| situated cells representing the covering layer (Rauber's layer)
| |
| of the Eutherian blastocyst. ‘'For this view,†remarks
| |
| Assheton [^09, p. 283), “1 can see no reason derivable from
| |
| actual specimens described and figured by those four authorsâ€
| |
| (Caldwell, Semon, Wilson and Hill), with which criticism I
| |
| am in entire agreement, as also with the following statement,
| |
| which, so far as the Metatheria are concerned, is based on
| |
| my own results: “Neither in the Prototheria [n ] or the
| |
| Metatheria is there really any tangible evidence of a trophoblast occui*ring as a covering layer over the definitive epiblast
| |
| as in Eutheria†(p. 234).
| |
| | |
| In connection with the peripheral growth of the unilaminar
| |
| blastoderm in the Monotreme, it is of interest to observe that
| |
| this takes place, not apparently in intimate contact with the
| |
| surface of the solid yolk, as is the case with the growing
| |
| margin of the extra-embryonal ectoderm in the Saui'opsidan
| |
| egg, but rather in contact with the inner surface of the
| |
| thickened zona, perhaps as the result of the accumulation in
| |
| the perivitelline space of tiuid which has diffused into the latter
| |
| from the uterus. In other words, the peripheral growth of
| |
| the extra-embryonal ectoderm to enclose the yolk-mass appears
| |
| to take place here in precisely the same way as the spreading
| |
| of the non-formative cells in Dasyurus to complete the lower
| |
| pole of the blastocyst. In my view the latter phenomenon
| |
| is none other than a recapitulation of the former ; on the
| |
| other hand, I regard the spreading of the formative cells in
| |
| Dasyurus towards the upper pole as a purely secondary
| |
| feature, conditioned by the loss of the yolk-mass and the
| |
| attainment of the holoblastic type of cleavage.
| |
| | |
| If it be admitted that the outer extra-embryonal layer of
| |
| the Monotreme blastocyst is homogenous with the extra
| |
| | |
| 92
| |
| | |
| | |
| J. P. HILL.
| |
| | |
| | |
| embryonal ectoderm of the Keptile, then it seems to me there
| |
| is no escape from tlie conclusion that these layers are also
| |
| homogenous with the non-formative region of the unilaminar
| |
| Marsupial blastocyst. I need only point out here that the
| |
| chief destiny of each of the mentioned layers, and I might
| |
| also add that of the outer enveloping layer of the Eutherian
| |
| blastocyst (the so-called trophoblast), is one and the same,
| |
| viz. to form the outer layer of the chorion (false amnion,
| |
| serous membrane) and omphalopleure (unsplit yolk-sac wall.
| |
| Hill ['97]),^ and that to deny their homogeny to each other
| |
| implies the nou-homogeny of these membranes and the amnion
| |
| in the Amniotan series, and consequently renders the group
| |
| name Amniota void of all moi'phological meaning.
| |
| | |
| The rapidity Avith which the enclosure of the yolk-mass
| |
| is effected, and the relative tardiness of differentiation in the
| |
| embryonal region are features Avhich sharply distinguish the
| |
| early ontogeny of the Monotremes from that of the Sauropsida,
| |
| and which, in my view, are of the very greatest importance,
| |
| since they afford the key to a correct understanding of the
| |
| peculiar coenogeuetic modifications observable in the early
| |
| ontogeny of the Metatheria and Eutheria. To appreciate the
| |
| significance of these featui-es it is necessary to take account
| |
| of the great difference which exists between the Sauropsidan
| |
| and Monotreme ovum in regard to size, as Avell as of the very
| |
| different conditions under Avhich the early development goes
| |
| on in the two groups. The Sauropsidan egg is large enough
| |
| to contain Avithin its OAvn confines the amount of yolk necessary for the production of a young one complete in all its
| |
| parts and capable of leading an independent existence
| |
| immediately it leaves the shell. Furthermore, it is also large
| |
| | |
| ' In certain Ainniotes the layers in question appear also to participate
| |
| in the formation of the inner lining of the amnion (amniotic ectoderm)
| |
| (cf . Assheton ['09], pp. 248-9), but this does not affect the statement in
| |
| the text. In the Saxu'opsida and Monotremata I think I am coia-ect in
| |
| saying that no sharp distinction is recognisable between the embi'yonal
| |
| and extra-embryonal regions of the ectoderm, hence it is difficult, if not
| |
| imj)ossible, to determine with certainty their relative participation in
| |
| the formation of the amniotic ectoderm.
| |
| | |
| | |
| THE EARLY DEVELOPMENT OP THE JIARSUPIALIA. 93
| |
| | |
| | |
| enough to provide room for tlie development of an embryo
| |
| without any secondary growth in size after it leaves the ovary.
| |
| Moreover we have to remember that after it has become
| |
| enclosed in the shelly it remains but a short time in the oviduct
| |
| and receives little or no additional nutrient material from the
| |
| oviducal walls. The yolk-mass in any case retains its solid
| |
| character; there is no necessity for its rapid enclosure, and
| |
| so enclosure is effected slowly, contemporaneously with the
| |
| differentiation of the embryo.
| |
| | |
| In the Monotreme the conditions are altogether different.
| |
| The ripe ovarian ovum when it enters the oviduct has a
| |
| diameter of about 3-5 to 4 mm., and is thns considerably
| |
| smaller than that of a Eeptile of the same size as the adult
| |
| Monotreme. The amount of yolk which it is capable of containing is not anything like sufficient to last the embryo
| |
| throughout the developmental period, and, moreover, it does
| |
| not provide the space essential for the development of an
| |
| embryo on the ancestral Reptilian lines. As Assheton ('98,
| |
| p. 251) has pointed out, “ the difference in size between
| |
| the fertilised ovum of a reptile or bird or of a mammal
| |
| is very great ; but the difference in size between the
| |
| embryo of, say, a bird with one pair of mesoblastic
| |
| somites and of a mammal of the same age is comparatively
| |
| small. This means that nearly the same space is required
| |
| for the production of the mammalian embryo as of the
| |
| Sauropsidan, and has to be provided.†In the Monotreme
| |
| not only is additional room necessary, but also additional
| |
| nutrient material, sufficient with that already present in the
| |
| egg to last the embryo throughout the period of incubation.
| |
| Both are acquired contemporaneously during the sojourn of
| |
| the egg in the uterine portion of the oviduct, wherein the egg
| |
| increases greatly in size. When it enters the uterus, the
| |
| Monotreme egg has a diameter, inclusive of its membranes, of
| |
| about 4-5 mm. ; when it is laid, it measures in Ornithorhynchus, in its greatest diameter, 16-19 mm., and somewhat
| |
| less in the case of Echidna. Prior to the enclosure of the yolk
| |
| the increase in diameter, due to the accumulation of fluid in
| |
| | |
| | |
| 94
| |
| | |
| | |
| J. P. HILL.
| |
| | |
| | |
| the perivitelliue space and between the zona and shell, is but
| |
| slight. But as soon as the yolk becomes suiTonnded by a
| |
| complete cellular membrane, i.e. as soon as the egg has
| |
| become converted into a thin-walled blastocyst, rapid growth
| |
| sets in, accompanied by the active imbibition of the nutrient
| |
| fluid, which is poured into the uterine lumen as the result of
| |
| the secretory activity of the abundantly developed uterine
| |
| glands. The fluid absorbed not only keeps the blastocyst
| |
| turgid, but it brings about the more or less complete disintegration of the yolk-mass, its constituent spherules
| |
| becoming disseminated in the fluid contents of the blastocyst
| |
| cavity. Although a distinct and continuous subgerminal
| |
| cavity, such as appears beneath the embryonal region of the
| |
| Sauropsidan blastoderm, does not occur in the Monotreme
| |
| egg, vacuolar spaces filled with fluid develop in the white
| |
| yolk-bed underlying the site of the germinal disc and appear
| |
| to represent it. As Wilson and Hill remark ('03, p. 317),
| |
| “ one can, without hesitation, homologise the interior of the
| |
| vesicle with the subgerminal cavity of a Saui'opsidan egg,
| |
| extended so as to include by liquefaction the whole of the
| |
| yolk itself.†In the Marsupial the blastocyst cavity has a quite
| |
| different origin, since it represents the persistent segmentation
| |
| cavity, whilst in the Eutheria the same cavity is secondarily
| |
| formed by the confluence of intra- or intei*-cellular vacuolar
| |
| spaces, but no one, so far as I know, has ever v^entured to
| |
| assert that, because of this difference in mode of origin, the
| |
| blastocyst cavity in the series of the Mammalia is a nonhomogenous formation.
| |
| | |
| To return to the matter under discussion, it appeal's to me
| |
| that the necessity which has arisen, consequent on the I'eduction in size of the ovum, for rapid growth of the same in
| |
| order to provide room for the development of an embryo and
| |
| for the storage of nutrient material furnished by the maternal
| |
| uterus, affords a satisfactory explanation of the much more
| |
| marked activity of the extra-embryonal I'egion of the blastoderm as compared with the embryonal, Avhich is such a striking
| |
| feature in the early ontogeny of the Monotremes, and not
| |
| | |
| | |
| THE EARLY DEVELOPMENT OF THE MARSUPIALIA. 95
| |
| | |
| | |
| only of them, but, as Assheton has pointed out ('98, p. 251),
| |
| of the higher mammals as well (cf. the process of epiboly and
| |
| the inertness at first displayed by the formative cells of
| |
| the embryonal knot as compared with the activity of the nonformative or tropho-ectodermal cells), an activity which
| |
| results in the rapid completion of that characteristically
| |
| mammalian developmental stage - the blastocyst or blastodermic vesicle.
| |
| | |
| The necessity for the early formation of such a stage,
| |
| capable of rapidly growing in a nutrient fluid medium
| |
| provided by the mother, has profoundly influenced the early
| |
| ontogeny in all three mammalian subclasses, and natui*ally
| |
| most of all that of the Eutheria, in which reduction of the
| |
| ovum, both as regards size and secondary envelopes, has
| |
| reached the maximum. And I think there can be little
| |
| doubt but that it is this necessity which has induced that
| |
| early separation of the blastomeres into two categories,
| |
| respectively formative and non-formative in significance,
| |
| which has long been recognised as occurring in Eutheria, and
| |
| which I have shown also occurs amongst the Metatheria.
| |
| This early separation of the blastomeres into two distinct
| |
| groups is not recognisable in the Sauropsida, and the idea
| |
| that it is in some way connected with the loss of yolk which
| |
| the mammalian ovum has suffered in the course ofphylogeny,
| |
| was first put forward, I believe, by Jenkinson. In his paper
| |
| on the germinal layers of Vertebrata ('06, p. 51) he writes:
| |
| “ Segmentation therefore is followed in the Placentalia by
| |
| the separation of the elements of the trophoblast from those
| |
| destined to give rise to the embryo and the remainder of its
| |
| foetal membranes, and this ^precocious segregation'
| |
| seems to have occurred phylogenetically during
| |
| the gradual loss of yolk which the egg of these
| |
| mammals has undergone.†Whether or not such a
| |
| precocious segregation †has already become fixed in the
| |
| Monotremes,future investigation must decide (cf . ante, p.90).
| |
| | |
| Ihe loss of yolk, with resulting reduction in size which the
| |
| Monotreme ovum has suffered in the course of phylogeny, we
| |
| | |
| | |
| 96
| |
| | |
| | |
| J. P. HILL.
| |
| | |
| | |
| must assume to have taken place gi-adually and in correlation
| |
| with the longer retention of the egg in the oviduct, the
| |
| elaboration of the uterine portion of the same as an actively
| |
| secretory organ, and the evolution of the mammary apparatus.
| |
| The Monotremes thus render concrete to us one of the first
| |
| great steps in mammalian evolution so far as developmental
| |
| processes are concerned, viz. the substitution for intra-ovular
| |
| yolk of nutrient material furnished directly by the mother to
| |
| the developing egg or embryo. We see in them the beginnings of that process of substitution of uterine for ovarian
| |
| nutriment which reaches its culmination in the Eutheria with
| |
| their microscopic yolk-poor ova and long intra-uterine period
| |
| of development. The Marsupials show us in Dasyurus an
| |
| interesting intervening stage so far as the ovum is concerned,
| |
| in that this, though greatly reduced as compared with that
| |
| of the Monotreme, still retains somewhat of its old tendencies
| |
| and elaborates more yolk-material than it can conveniently
| |
| utilise, with the result that it has to eliminate the surplus
| |
| before cleavage begins. But as coucerns their utilisation of
| |
| intra-uterine nutriment, they have specialised along their
| |
| own lines, and instead of exhausting the possibilities implied
| |
| by the presence of that, they have extensively elaborated
| |
| the mammary apparatus for the nutrition of the young, born
| |
| in a relatively immature state, after a short period of intrauterine life (cf. Wilson and Hill [T7, p. 580]).
| |
| | |
| In view of the fact that the young Monotreme enjoys three
| |
| developmental periods, viz. intra-uterine, incubatory, and
| |
| lactatory, the question might be worthy of consideration
| |
| whether it may not be that the Marsupial has merged the
| |
| incubatory period in the lactatory, the Eutherian the same in
| |
| the intra-uterine.
| |
| | |
| | |
| 2. The Early Development of the Metatheria and
| |
| | |
| Eutheria.
| |
| | |
| It will have become evident Horn the foregoing that the
| |
| Metatherian mode of early development is to be regarded as
| |
| | |
| | |
| THE EARLY DEVELOPMENT OP THE MAESUPIALIA.
| |
| | |
| | |
| 97
| |
| | |
| | |
| but a slightly modified version of the Prototherian, such
| |
| differences as exist between them being interpretable as coenogeuetic modifications, induced in the Metatherian by the
| |
| practically complete substitution of uterine nutriment for
| |
| intra-ovular yolk, a substitution which has resulted in the
| |
| attainment by the marsupial ovum of the holoblastic type of
| |
| cleavage. In tlie present section I hope to demonstrate how
| |
| the early ontogeny of the Metatlieria enables us to interpret
| |
| that of the Eutheria in terms of that of the Prototheria.
| |
| | |
| If we proceed to compare the early development in the
| |
| Metatlieria and Eutheria, we encounter, from the 4-celled
| |
| stage onwards, such obvious and profound differences in the
| |
| mode of formation of the blastocyst, and in the relations of
| |
| its constituent parts, that the differences seem at first sight
| |
| to far outweigh the resemblances. Nevertheless, apart from
| |
| their common possession of the same holoblastic mode of
| |
| cleavage, there exists one most striking and fundamental
| |
| agreement between the two in the fact that in both there
| |
| occurs, sooner or later during the cleavage process, a separation of the blastomeres into two distinct, pre-determined cellgroups, whose individual destinies are very different, but
| |
| apparently identical in the two subclasses. In tlie Marsupial,
| |
| as typified by Dasyurus, the fourth cleavages are, as we have
| |
| seen, unequal and qualitative, and result in the separation of
| |
| two differentiated groups of blastomeres, arranged in two
| |
| superimposed rings, viz. an upper ring of eight smaller, less
| |
| yolk-rich cells, and a lower of eight larger, more yolk-iuch
| |
| cells. The evidence justifies the conclusion that the former
| |
| gives origin directly to the formative or embryonal region of
| |
| the vesicle wall, the latter to tlie non-formative or extraembryonal region.
| |
| | |
| Amongst the Eutheria the evidence is no less clear. It has
| |
| been conclusively shown by various observers (Van Beneden,
| |
| Duval, Assheton, Hubrecht, Heape, and others) that, sooner
| |
| or later, there occui's a separation of the blastomeres into two
| |
| distinct groups, one of which eventually encloses the other
| |
| completely. The two groups may be clearly distinguishable
| |
| | |
| VOL. 56, PART 1. NEW SERIES. 7
| |
| | |
| | |
| 98
| |
| | |
| | |
| t.C.TTV. (€cj
| |
| | |
| trect.
| |
| | |
| | |
| i.c.nv.ffo.)
| |
| | |
| | |
| | |
| tr.ect.
| |
| | |
| | |
| cunrvTh.c.
| |
| emJb. ect.
| |
| | |
| | |
| Diagrams illustrating the mode of formation of the blastocyst
| |
| in Metatheria (a-d) and Eutheria (1-3). b.c. Blastocyst cavity.
| |
| i.c.m. Inner cell-mass, 'pr.amn.c. Primitive amniotic cavity.
| |
| r.l. Rauber's layer. s.c. Segmentation cavity. For other
| |
| reference letters see explanation of plates (p. 125).
| |
| | |
| | |
| THE EAltLY l.)E VET;01â– '^[ENâ– T OF THE MARSUFIALIA. 99
| |
| | |
| | |
| in eai'lv cleavage stages, owing to diffecences in the characters
| |
| and staining reactions of their cells, and in such cases there
| |
| is definite evidence of the occurrence of a process of overgrowth
| |
| or epiboly, whereby one group gradually grows round and
| |
| completely envelops the other, so that in the completed
| |
| morula a distinction may be drawn between a central cellmass and a peripheral or enveloping layer (rabbit. Van
| |
| Beneden; sheep, Assheton). In other cases, where it has
| |
| been impossible to recognise the existence of these two
| |
| distinct cell-groups in the cleavage stages, we nevertheless
| |
| find, either in the completed moimla or in the blastocyst, that
| |
| a more or less sharp distinction may be drawn between an
| |
| enveloping layer of cells and an internally situated cell-mass
| |
| (inner cell-mass).
| |
| | |
| E. van Beneden, in his classical paper on the development
| |
| of the rabbit, published in 1875, was the first to recognise
| |
| definitely the existence of two categories of cells in the
| |
| segmenting egg of the Eutherian mammal. In this form he
| |
| showed how in the morula stage a cap of lighter blastomeres
| |
| gradually grows round and envelops a mass of more opaque
| |
| cells by a process of overgrowth or epiboly. In his more
| |
| recent and extremely valuable paper on the development of
| |
| Yespertilio ('99), he again demonstrated the existence of two
| |
| groups of blastomeres as well in the segmenting egg as in the
| |
| completed morula, but failed to find evidence of epiboly in all
| |
| cases. Nevertheless he holds fast to the opinion which he
| |
| expressed in 1875 : “ Que la segmentation s'accompagne, chez
| |
| les Mammiferes placentaires, d'un enveloppement progressif
| |
| d'une partie des blastomeres par une couche cellulaire, qui
| |
| commence a se differencier des le debut du developpement,â€
| |
| and states that “dans tons les oeufs arrives a la fin de la
| |
| segmentation et dans ceux qui moutraient le debut de la
| |
| cavite Blastodermique j'ai constamment rencontre une couche
| |
| peripherique complete, eutourant de toutes parts un amas
| |
| cellulaire interne, bien separe de la couche enveloppante.â€
| |
| The latter layer he regards as corresponding to the extraembryonal ectoderm of the Sauropsida, and points out that
| |
| | |
| | |
| 100
| |
| | |
| | |
| J. P. HILL.
| |
| | |
| | |
| “ chez tons les Choi'des les premiers blastomeres qui se
| |
| differencient et qui avoisinent le pole animal de I'oeuf sont
| |
| des elements epiblastiqnes. C'est par la couolie cellulaire qui
| |
| resulte de la segmentation ulterieure de ces premiers blastomeres epiblastiqnes que se fait, cbez les Sauropsides, benveloppement du vitellus. Dans Toeuf reduit a n'etre plus
| |
| qu'une sphere microscopiquej bepibolie a pu s'achever des la
| |
| fin de la segmentation, voire meme avant bachevement de ce
| |
| phenomene.†The “ amas cellulaire interne †(embryonal
| |
| knot, inner cell mass). Van Beneden shows, differentiates
| |
| secondarily into “ un lecithophore et un bouton embryonnaire.'' The former is the entoderm of other authors, the
| |
| latter the formative or embryonal ectoderm. Hubrecht, in
| |
| the forms studied by him (Sorex, 'I'upaia, Tarsius^) finds
| |
| a corresponding differentiation. In Tupaia he describes the
| |
| morula stage as consisting of a single central lightly staining
| |
| cell, which he regards as the parent cell of the inner cell-mass
| |
| of later stages, and of a more darkly staining peripheral layer
| |
| which forms the unilaminar wall of the blastocyst. Here,
| |
| then, the parent cells of the two cell-groups would appear to
| |
| be separated at the first cleavage. Hubrecht, like Van
| |
| Beneden, holds that the inner cell-mass furnishes the
| |
| embryonal ectoderm and the entire entoderm of the blastocyst.
| |
| The peripheral layer he has termed the trophoblast ('88, p.
| |
| 511), and in his paper on the placentation of the hedgehog
| |
| ('89, p. 298) he defines the term as follows: “I propose to
| |
| confer this name to the epiblast of the blastocyst as far as it
| |
| has a dix'ect nutritive significance, as indicated by proliferating
| |
| processes, by immediate contact with maternal tissue, maternal
| |
| blood, or secreted material. The epiblast of the germinal
| |
| ai-ea - the formative epiblast - aud that which will take part
| |
| in the formation of the inner lining of the amnion cavity is,
| |
| ipso facto, excluded from the definition.†Thus the name
| |
| | |
| * In Erinacens the entoderm, from Hubrecht's observations, appears
| |
| to be precociously differentiated, prior to the separation of the embryonal
| |
| ectoderm fi'om the overlying trophoblast, but the details of the early
| |
| development in this form are as yet only incompletely known.
| |
| | |
| | |
| THE EARLY DEVELOPMENT OP THE MARSHPIALIA. 101
| |
| | |
| | |
| trophoblasb was originally employed by Hubrecbt as a convenient term designatory of what he at the time regarded as
| |
| the extra-embryonal ectoderm of the mammalian blastocyst.
| |
| In the course of his speculations on the oingin of this layer,
| |
| however, he has reached the conclusion that it is really of the
| |
| nature oP'a larval envelope, an Embryonalhiille (^08, p. 15),
| |
| inherited by the mammals, not from the reptiles (which have
| |
| no direct phylogenetic I'elationship to the latter), but from
| |
| their remote invertebrate ancestors ('Vermiform pi'edecessors
| |
| of coelenterate pedigree, provided with an ectodermal larval
| |
| investment [Laiwenhiille] â€).
| |
| | |
| Assheton, again, although he was unable to convince himself ('94) of the correctness of van Beneden's account of the
| |
| occurrence of a process of epiboly in the segmenting eggs of
| |
| the rabbit, finds in the sheep ('98) that a differentiation into
| |
| two groups of cells is recognisable “ perhaps as early as the
| |
| eight segment stage,†and that one of the groups gradually
| |
| envelops the other. “Let it be noted,†he writes ('98, p. 227),
| |
| “ that we have now to face the fact, based on actual sections,
| |
| that there is in certain mammals a clear separation of
| |
| segments at an early stage into two groups, one of which
| |
| eventually completely surrounds the other,†and instances
| |
| Van Beneden's observations on the rabbit (of the correctness
| |
| of which he, however, failed to satisfy himself, as noted above),
| |
| Duval's observations on the bat, Hubrecht's on Tupaia, and
| |
| his own on the sheep. Assheton thinks this phenomenon
| |
| “ must surely have some most profound significance,â€
| |
| but finds himself unable to accept the interpretations of
| |
| either Van Beueden or Hubrecht, and puts forward yet
| |
| another view, “ based on the appearance of some segmenting
| |
| eggs of the sheep †('08, p. 233), “that in cases where this
| |
| differentiation does clearly occur, it is a division into epiblast
| |
| and hypoblast, the latter being the external layer†('98, p. 227).
| |
| Assheton thus differs from all other observers in holding that
| |
| the inner cell-mass or embryonal knot of the Eutherian
| |
| blastocyst gives origin solely to the formative or embryonal
| |
| ectoderm, and I believe 1 am correct in stating that he also
| |
| | |
| | |
| 102
| |
| | |
| | |
| J. p. mi,L.
| |
| | |
| | |
| differs from all other observers in holding that the outer
| |
| enveloping layer of the same is entodermald
| |
| | |
| The fact, then, of the occurrence amongst Eutheria of a
| |
| “precocious segregation †of the blastomeres into two distinct
| |
| groups, one of which eventually surrounds the other completely, is not in dispute, though authorities differ widely in
| |
| the intei'pretation they place upon it. In the Eutherian
| |
| blastocyst stage, the enveloping layer forms the outer unilaminar wall of the vesicle, and encloses the blastocyst cavity
| |
| as well as the other internally situated group. This latter
| |
| typically appears as a rounded cell-mass, attached ac one spot
| |
| to the inner surface of tlie enveloping layer, but more or less
| |
| distinctly marked off from it. It is genei-ally termed the
| |
| inner cell-mass or embryonal knoc (“ amas cellulaire interne â€
| |
| of Van Beneden). For the enveloping layer Ilubrecht's name
| |
| of “ trophoblast †is now generally employed, even by those
| |
| who refuse to adopt the speculative views with which its
| |
| originator has most unfortunately, as I think, enshrouded this
| |
| convenient term.
| |
| | |
| I have demonstrated the occurrence of an apparently comparable “precocious segregation^^ of the blastomeres into
| |
| two distinct groups in one member of the Metatheria which
| |
| there is no reason to regard as an abeirant type, and I have
| |
| shown beyond all shadow of doubt that from the one group,
| |
| which constitutes what I have termed the formative region
| |
| of the unilaminar vesicle-wall, there arise the embi*youal
| |
| ectoderm and the entire entoderm of the vesicle, both embryonal and extra-embryonal, and that the other group, which
| |
| constitutes the non-formative region of the vesicle-wall,
| |
| directly furnishes the extra-embryonal ectoderm, i.e. the
| |
| ectoderm of the omphalopleui'e and chorion."
| |
| | |
| * Assheton states ('08, p. 233, cf. also '98, p. 220) that his interpretation “ owes ranch also to the theoretical conclusions of Minot and
| |
| Robinson.†However that may be, both Minot and Robinson in their
| |
| most recent writings continue to speak of the chorionic ectoderm.
| |
| | |
| ^ Whether or not it participates in the formation of the ainniotic
| |
| ectoderm future investigation must decide.
| |
| | |
| | |
| THE EARLY DEVELOPMENT OF THE MARSUPFALIA. 103
| |
| | |
| As resrards Eutheria, we have seen that Van Beneden and
| |
| Hubrecht, though their views in otlier respects are widely
| |
| divero-ent, both ag'ree that the inner cell-mass of the blastocyst furnishes the embryonal ectoderm (as well as the amniotic
| |
| ectoderm wholly or in part) and the entire entoderm of the
| |
| vesicle. That, in fact, is the view of Mammalian embryologists
| |
| generally (Duval and Assheton excepted),^ and if we may
| |
| assume it to be correct, then it would appear that the later
| |
| history of the formative region of the Marsupial blastocyst
| |
| and that of the inner cell-mass of the Eutherian are identical.
| |
| That being so, and bearing in mind that both have been
| |
| shown, at all events in certain Mammals, to have an identical
| |
| origin as a group of precociously segregated blastotneres,^ I
| |
| can come to no other conclusion than that they are homogenous formations. If that be accepted, then this fact by itself
| |
| renders highly probable the view that the so-called trophoblast of the Eutherian blastocyst is homogenous with the
| |
| non-formative region of the Metatherian vesicle, and v?hen
| |
| we reflect that both have precisely the same structural and
| |
| topographical (not to mention functional) relations in later
| |
| stages, inasmuch as they constitute the ectoderm of the chorion
| |
| and omphalopleure (with or without participation in the
| |
| formation of the amniotic ectoderm;, and that both have a
| |
| similar origin in those Mammals in which a precocious segregation of the blastomeres has been recognised, their exact
| |
| | |
| * The view of Duval ['95], based on the study of Vespertilio, that the
| |
| inner cell-mass gives rise solely to entoderm, and that the enveloping
| |
| layer furnishes not only the extra-embryonal but also the embryonal
| |
| ectoderm, is shown by Van Beneden's observations on the same form to
| |
| be devoid of any basis of fact. Assheton's views are referred to below
| |
| (p. 110).
| |
| | |
| - The fact that the phenomenon of the “ precocious segregation†of
| |
| the blastomeres into two groups with deteiminate destinies has already
| |
| become fixed in tlie Marsupial lends additional weight to the view of
| |
| Van Beneden that such a segregation will eventually be recognised as
| |
| occurring in all Eutheria without exception. Without it, it is difficult
| |
| to understand how the entypic condition, characteristic of the blastocysts of Ml known Eutheria, is attained, imless by differentiation in
| |
| situ, which .seems to me highly improbable.
| |
| | |
| | |
| 104
| |
| | |
| | |
| J. r. HILL.
| |
| | |
| homology need no longer be doubted. In the preceding section
| |
| of this paper (ante, pp. 91, 92) I have shown reason for the
| |
| conclusion that the non-formative region of the Marsupial
| |
| blastocyst is the homologue of the extra-embryonal ectoderm
| |
| of the Monotreme and Reptile, and if that conclusion be
| |
| accepted it follows that the outer enveloping layer of the
| |
| Eutherian blastocyst, the so-called trophoblast of Hubrecht,
| |
| is none other than extra-embryonal ectoderm, as maintained
| |
| by Van Beneden, Keibel, Bonnet, Jenkinson, Lee, MacBride
| |
| and others, the homologue of that of Reptilia.
| |
| | |
| I am therefore wholly unable to accept the highly speculative conclusions of Hubrecht, set forth with such brilliancy
| |
| in a comparatively recent number of this Journal ('08), as
| |
| to the significance and phylogeny of this layer. These conclusions, on the basis of which he has proceeded to formulate
| |
| such far-reaching and, indeed, revolutionary ideas not only
| |
| on questions embryological, but on those pertaining to the
| |
| phylogeny and classification of vertebrates, have already
| |
| been critically considered by Assheton ('09) and MacBride
| |
| ('09), also in the pages of this Journal, and found wanting,
| |
| and they are, to my mind, quite irreconcilable with the facts
| |
| I have brought to light in regard to the early development
| |
| of Marsupials. I yield to no one in my admiration for the
| |
| epoch-making work of Hubrecht on the early ontogeny and
| |
| placentation of the Mammalia, and I heartily associate
| |
| myself with the eulogium thereanent so admirably expressed
| |
| by Assheton in the cx'itique just referred to (p. 274), but
| |
| I am bound to confess that as concerns his views on the
| |
| phylogeny of this layer, which he has termed the “ trophoblast,†he seems to me to have forsaken the fertile field of
| |
| legitimate hypothesis for the barren waste of unprofitable
| |
| speculation, and to have erected therein an imposing edifice on
| |
| the very slenderest of foundations.
| |
| | |
| Before I proceed to justify this, my estimate of Hubrecht's
| |
| views on the phylogeny of the trophoblast, let me first set
| |
| forth his conception so far as I understand it. He starts
| |
| with the assumption that the vertebrates (with the exception
| |
| | |
| | |
| THE EARLY EEVELOrMENT OF THE MARSUPIALIA. 105
| |
| | |
| | |
| of Ainpliioxus, the CyclostoineSj and the Elasraobi'anclif!) are
| |
| descended from “vermiform predecessors of coelenterate
| |
| pedigree†possessed of free-swimming larvte, in which there
| |
| was present a complete larval membi'ane of ectodermal derivation, and of the same order of differentiation “as the outer
| |
| larval layer which in certain Nemertines, Gephyreans, and other
| |
| worms often serves as a temporaiy envelope that is stripped
| |
| off when the animal attains to a certain stage of development.â€
| |
| When, for oviparity and larval development, viviparity and
| |
| embryonic development became established in the Protetrapodous successors of the ancestral vermiform stock, the
| |
| larval membrane did not disappear. On the contrary, it is
| |
| assumed that it merely changed “its protective or locomotor
| |
| function into an adhesive one,†and so, development now
| |
| taking place in utero, it is quite easy to understand how tlie
| |
| larval membrane could gradually become transformed into
| |
| a trophic vesicle, containing the embryo as before, and
| |
| functional in the reception of nutriment from the walls of
| |
| the maternal uterus. The final stages in the evolution of
| |
| this trophic vesicle constituted by the old larval membrane
| |
| are met with amongst the mammals, since in them it
| |
| became vascularised so as to constitute a “yet more
| |
| thorough system of nourishment at the expense of the
| |
| maternal circulatory system.†Such, then, is the phylogeny
| |
| of the trophoblast according to Hubrecht. The Eutheriau
| |
| mammals, which it is held trace their descent straight back to
| |
| some very early Protetrapodous stock, viviparous in habit and
| |
| with small yolk-poor, holoblastic eggs, exhibit the trophoblast in its most perfect condition. Hubrecht therefore starts
| |
| with them, and attempts to demonsti'ate the existence of a
| |
| larval membrane, or remnants of such, externally to the
| |
| embryonal ectoderm in all vertebrates with the exceptions
| |
| already mentioned. There is no question of its existence in
| |
| the Meta- and Eutherian mammals. “We may,†writes
| |
| Hubrecht ('08, p. 12), . . . “insist upon the fact that
| |
| | |
| . . . all Didelphia and Monodelphia hitherto investi
| |
| gated show at a very early moment the didermic stage out of
| |
| | |
| | |
| 106
| |
| | |
| | |
| J. P. HILL.
| |
| | |
| | |
| which the embryo will be built up enclosed in a cellular
| |
| vesicle (the troplioblast), of which no pai‘t ever enters into
| |
| the embryonic organisation.†The common possession by the
| |
| Metatheria and Eutheria of a larval membi'ane is after all
| |
| only what might be expected, “since after Hill's ('97)
| |
| investigations, we must assume that the didelphian mammals
| |
| are not descended from Ornithodelphia but from monodelphian
| |
| placental ancestors.†As concerns the Prototheria, although
| |
| they cannot in any sense be regarded as directly ancestral to
| |
| the other mammals, we nevertheless find the trophoblastic
| |
| vesicle “ compax'atively distinct.†“In many reptiles and
| |
| birds,†however, it is “.distinguished with great diflSculty
| |
| from the embryonic shield,†and this is explained bv the
| |
| fact that the Sauropsida which are assumed to have taken
| |
| their origin from the same Protetrapodous stock as the
| |
| mammals but along an entirely independent line, have
| |
| secondarily acquired, like the Prototheria, the oviparous
| |
| habit, with its concomitants, a yolk-laden egg and a shell, and
| |
| this latter acquisition has naturally tended “to relegate any
| |
| outer larval layer to the pension list†('09, p. 5). “Concerning the yolk accumulation in the Sauropsidan egg, there
| |
| is no trouble at all to suppose that the vesicular blastocyst
| |
| of an early vivipai-ous ancestor had gradually become yolkladen. The contrary assumption, found in the handbooks,
| |
| that the mammalian egg, while totally losing its yolk, has
| |
| yet preserved the identical developmental featui-es as the
| |
| Sauropsid, is in ideality much more difiicult to reconcile with
| |
| sound evolutionary principles†('09, p. 5).
| |
| | |
| Amongst the lower Vertebrates the larval membrane is
| |
| clearly enough recognisable in the so-called Deckschicht of
| |
| the Teleostomes, Dipnoans, and Amphibians. It is frankly
| |
| admitted that Amphioxus, the Cyclostomes, and the Elasmobranchs “ show in their early development no traces of a
| |
| Deckschicht†(larval layer, troiDhoblast), but there is no
| |
| difficulty about this, since it is easy enough to suppose, in
| |
| view of other characters, that “ the Selachians may very well
| |
| have descended from ancestors without any outer larval layer â€
| |
| | |
| | |
| THE EAKLY HEVELOrMENT OL<' THE MAESOPIALIA. 107
| |
| | |
| {'08, p. 151), and ‘'for Cyclostomes tlie same reasoning holds
| |
| good†(p. 152).
| |
| | |
| The trophoblast, then, is conceived of by Hubrecht as a
| |
| larval membrane of ectodermal derivation, which invests the
| |
| embryonal ahlage in all Vertebrates with the exceptions
| |
| mentioned, 'which is subject to secondary reduction, and which
| |
| is homologous throughout the series. As I understand the
| |
| conception, what is ordinarily called extra-embryonal ectoderm in the Sauropsida is not trophoblast, otherwise Hubrecht
| |
| could hardly write - “in reptiles and birds traces of the
| |
| larval layer have in late years been unmistakably noticedâ€
| |
| ('09, p. 5) ; nevertheless what other writers have termed
| |
| embryonal and extra-embryonal ectoderm in the Prototheria
| |
| is claimed by Hubrecht as trophoblast (at all events that is
| |
| my interpretation of his statement that a trophoblastic vesicle
| |
| is present in these forms), and yet some years ago Hubrecht
| |
| ('04, p. 10) found it diflBcult “ to understand that the name
| |
| has been misunderstood both by embryologists and gynecologists.†My own feeling is that the more recent developments in his views have tended to obscure rather than to
| |
| clarify our ideas as to the trophoblast, especially if we must
| |
| now hold that the chorion or serosa of the Sauropsida is not
| |
| homologous with that of the Prototheria, which necessarily
| |
| follows if the extra-embi'yonal ectoderm of the Sauropsidan is
| |
| not the same thing as that of the Monotreme.
| |
| | |
| Assuming that we have formed a correct conception of the
| |
| trophoblast as a larval membrane, and bearing in mind that it
| |
| is best developed in the Metatheria and Eutheria, since these
| |
| alone amongst higher Vertebrates have retained unaltered
| |
| the viviparous habits of their Protetrapodous ancestors, let us
| |
| see what basis in fact there is for the statement of Hubrecht
| |
| ('08, p. 68) that “before the ectoderm and the entoderm
| |
| have become differentiated from each other there is in
| |
| mammals a distinct larval cell-layer surrounding (as soon as
| |
| cleavage of the egg has attained the morula stage) the
| |
| mother-cells of the embryonic tissues.†Now that statement
| |
| as it stands, I have no hesitation in characterising as entirely
| |
| | |
| | |
| 108
| |
| | |
| | |
| .T. P. HlIiL.
| |
| | |
| | |
| misleading, inasmuch as it is applicable not to the Mammalia
| |
| as a whole, but, so far as it refers to matters of undisputed
| |
| fact, to one only of the three mammalian subclasses, viz. the
| |
| Eutheria. So far as the latter ai'e concerned, practically all
| |
| observers, as we have seen, are agreed that there is present
| |
| during at least the early stages of development a complete
| |
| outer layer of cells which encloses the embryonal anlage
| |
| or inner cell-mass (that portion of it immediately overlying
| |
| the latter being termed the “ Deckschicht †or “Rauber's
| |
| layerâ€). It is, of course, this envelojDing layer or trophoblast which Hubrecht interprets as a larval membrane.
| |
| It fulfils the conditions, and were the Eutheria the only
| |
| Vertebrates known to us, the idea might be plausible
| |
| enough.
| |
| | |
| Turning now to the Metatheria, and I'emembering that these,
| |
| according to Hubrecht, are descended from the Eutheria, we
| |
| should naturally expect to find the supposed larval membrane
| |
| fully developed, with all its ancestral relations ; and so we do
| |
| if we are content to accept Hubrecht's interpretation of
| |
| Selenka's results and figures in the case of Didelphys. The
| |
| “ urentodermzelle †of Selenka is for Hubrecht “ undoubtedly
| |
| the mother-cell of the embryonic knob,†the ectoderm of
| |
| Selenka is manifestly the trophoblast - a complete larval
| |
| layer. It is no doubt unfortunate that Hubrecht had to rely
| |
| on the work of Selenka as his source of information on the
| |
| early development of Marsupials, but it must be remembered
| |
| that he reads his own views into Selenka's figures. On the
| |
| basis of my own observations on the early ontogeny of Marsupials, I have no hesitation in affirming that a larval membrane, in the sense of Hubrecht, does not exist in any of the
| |
| forms (Dasyurus, Perameles, Macropus) studied by me. The
| |
| observations recorded in the preceding pages of this paper
| |
| demonstrate, in the case of Dasyurus without the possibility
| |
| of doubt, the entire absence of any cellular layer external
| |
| to the formative region of the blastocyst, i.e. in a position
| |
| corresponding to that occupied by Rauber's layer in Eutheria,
| |
| whilst in the case of Perameles and Macropus, they yield not
| |
| | |
| | |
| THE EAULY DEVELOPMENT OP THE MAESUPIALTA. 109
| |
| | |
| | |
| the slightest evidence for the existence of any such layer.
| |
| The formative region of the Marsupial blastocyst, which is
| |
| undoubtedly the homologue of the inner cell mass of the
| |
| Eutheria, forms from the first part of the unilarninar blastocyst wall, and is freely exposed. The remainder of the latter
| |
| is constituted by a layer of non-formative cells, the destiny
| |
| of which is the same as that of the so-called trophoblast of
| |
| the Eutheria. I have therefore ventui'ed to suggest that they
| |
| are one and the same. If, then, the trophoblast is really a
| |
| larval membrane, we must assume, in the case of the Marsupial, either that its “ Deckschicht '' portion has been completely suppressed (but why it should have been I fail to
| |
| understand, unless, perhaps, it is a result of the secondary
| |
| acquisition by the Marsupials of a shell-membrane, these
| |
| mammals being even now on the, way to secondarily assume
| |
| the oviparous habit !), or that the non-formative region of the
| |
| Marsupials is not the homologue of the trophoblast, in which
| |
| case the Marsupials must be held to have entirely lost the larval
| |
| membrane, since there is no other layer present which could
| |
| possibly represent it. These considerations may well give us
| |
| pause before we calmly accept Hubrecht's conception of the
| |
| trophoblast as a larval membrane present in all mammals
| |
| without exception.
| |
| | |
| Coming now to the Prototheria, we find, according to
| |
| Hubrecht, the trophoblastic vesicle . . . yet compara
| |
| tively distinct,†and so it is if we accept the interpretation of
| |
| Hubrecht of the observations and figures of Semon, Wilson
| |
| and Hill. The unilarninar blastoderm of these authors is
| |
| unmistakably the trophoblast. The cells situated internally
| |
| to that in the region of the white yolk-bed are not entodertnal, as suggested by Semon, but constitute for Hubrecht
| |
| “ the mother cells of the embryonic knob.†I need only quote
| |
| again the opinion of Assheton thereanent and express my
| |
| agreement therewith; he writes (^09, p. 233) : For this view
| |
| | |
| I can see no reason derivable from actual specimens described
| |
| and figured by those four authors†(Caldwell, Semon, Wilson
| |
| and Hill). It would appear, then, that the assumption of
| |
| | |
| | |
| 110
| |
| | |
| | |
| J. P. HILL.
| |
| | |
| | |
| Hubreclit of the presence of a larval membrane of the nature
| |
| postulated in the Prototheria and Metatheria is devoid of
| |
| foundation in fact, so that there but remains the question of
| |
| the significance of the outer enveloping layer of the Eutherian
| |
| blastocyst. As regards that, I venture to think that the
| |
| alternative interpretation of E. van Beneden and other
| |
| investigators, which I have attempted to develop in the
| |
| pages of this paper, affords a simpler and more satisfying
| |
| explanation of its significance and phylogeny than that
| |
| advocated by Prof. Hubrecht, an interpretation, moreover,
| |
| which is more in accordance, not only with all the known
| |
| facts, but ''with sound evolutionary principles and with the
| |
| conclusions arrived at by the great majority of comparative
| |
| anatomists and palaeontologists as to the origin and intei-relationships of the Mammalia.
| |
| | |
| And I also venture to think that what has just been said
| |
| holds true with reference to the views advocated by Mr.
| |
| Assheton. These views owed their origin to certain appearances which he found in some segmenting ova of the sheep
| |
| (but, be it noted, not in all those he examined), and he has
| |
| attempted to re-intei pret not only his own earlier observations,
| |
| but those of other workers on the early ontogeny of the Eutheria
| |
| in the light of his newer faith, and not only so, he holds that it
| |
| is also possible to apply that in the interpretation of the early
| |
| ontogeny of Marsupials (v. '08, p. 235, and '09, p. 229). He
| |
| maintains that the inner cell-mass of Eutheria is purely ectodermal, aud that the enveloping trophoblast layer of the blastocyst arises in common with the entodermal lining of the same
| |
| and is therefore also entodei'mal. " On the theory I advocate,â€
| |
| he writes ('09, p. 235), " the trophoblast is of Eutherian
| |
| mammalian origin only and is not homologous to any form of
| |
| envelope outside the group of Eutherian mammals.†These
| |
| views of Assheton are not only at variance with those of all
| |
| other investigators who have worked at the early ontogeny of
| |
| Eutheria, but they are quite irreconcilable with my observations on the development of Dasyurus herein recorded. I claim
| |
| to have shown in that Marsupial that the formative region, the
| |
| | |
| | |
| THE EAELY DEVELOPMENT OP THE MAllSUPIALIA. Ill
| |
| | |
| | |
| homologneof the inner cell-mass, gives origin not only to the
| |
| embryonal ectoderm, but to the entire entoderm, whilst tlie
| |
| non-formative region, whose homology to the trophoblast of
| |
| Eutheria is admitted by Assheton, arises quite independently
| |
| of the entoderm and a long time before the latter inakes its
| |
| appearance. There is, then, in Dasyurus no question of a
| |
| common origin of the entoderm and the non-forrnative or
| |
| trophoblastic region of the blastocyst wall. And exception
| |
| inay be taken to Assheton's views on quite other grounds
| |
| (e. g. the question of the homologies of the foetal membranes
| |
| in the series of the Amniota), as he himself is well awai'e, and
| |
| as Jenkinson ('00) has also emphasised. I feel, however, I can
| |
| leave further discussion of Assheton's views until such time
| |
| as my observations on Dasyurus are shown to be erroneous or
| |
| inapplicable to other Marsupials.
| |
| | |
| 3. The Entypic Condition of the Eutherian
| |
| | |
| Blastocyst.
| |
| | |
| If, now, on the basis of the homologies I have ventm-ed to
| |
| advocate in the preceding pages, we proceed to compare the
| |
| Metatherian with the Eutherian blastocyst, we have to note
| |
| that, whereas in the latter the extra-embryonal or trophoblastic ectoderm alone forms the blastocyst wall in early
| |
| stages and completely encloses the embryonal knot, in the
| |
| former, the homologous parts, viz. the non-formative or exti'aembryonal and the formative or embryonal regions, both
| |
| enter into the constitution of the unilaminar blastocyst
| |
| wall, there being no such enclosure of the one by the
| |
| other as occurs in the Eutherian blastocy.st (Text-fig. 2, p. 98).
| |
| It is characteristic of the Marsupial as of the Monotreme that
| |
| the embryonal region is from the first superficial and freely
| |
| exposed. It is spread out as a cellular layer and simply
| |
| forms part of the blastocyst wall or blastoderm. It is equally
| |
| characteristic of the Eutherian that the homologous part, the
| |
| embryonal knot, has at first the form of a compact mass,
| |
| which is completely enclosed by the trophoblastic ectoderm.
| |
| | |
| | |
| 112
| |
| | |
| | |
| J. P. HILL.
| |
| | |
| | |
| The latter alone constitutes the unilaminar wall of the
| |
| blastocyst and has the embryonal knot adherent at one spot
| |
| to its inner surface. The formative cells which compose
| |
| the knot thus take at first no part in the constitution of
| |
| the outei wall of the blastocyst^ and may or may not
| |
| do so in later stages according as the covering layer of the
| |
| trophoblast (the Deckschicht or Rauber's layer) is transitory or permanent. This peculiar developmental condition, characterised by the internal position of the formative
| |
| or embryonal cells within the blastocyst cavity, has been
| |
| termed by Selenka (TO) “entypy†(Entypie des Keimfeldes).^ It is a phenomenon exclusively found in the
| |
| Eutheria and characteristic of them alone, amongst the
| |
| mammals. In the Marsupial, as in the Monotreme, the
| |
| formative cells are freely exposed, and constitute from the first
| |
| part of the blastocyst wall just as those of the Sauropsida form
| |
| a part of the general blastoderm. Limited as entypy thus
| |
| appears to be to the higher mammals, the probability is that
| |
| we have to do here with a purely secondary, adaptive feature.
| |
| | |
| If we proceed to inquire what is the significance of this
| |
| remarkable difference in the early developmental phenomena
| |
| of the lower and higher mammals, it seems to me that we have
| |
| to take account, in the first place, of the differences in the
| |
| structure of their respective eggs, and especially we have to
| |
| bear in mind that the Eutherian ovum is considerably more
| |
| specialised than even the Metatherian. It is on the average
| |
| smaller than the latter, i.e. it has suffered in the course of
| |
| phytogeny still further reduction in size, and has lost, to an
| |
| even greater extent than the Marsupial ovum, the store of foodyolk ancestrally present in it. Moreover, it has suffered a still
| |
| further i-eduction in respect of its secondary egg-membranes.
| |
| The Metatherian ovum still retains in its shell-membrane a
| |
| | |
| ^ “ Unter Entypie des Keimfeldes mdcbte ich dalier verstanden
| |
| wissen : Die nicht dm-cli Bildung typischer Anmionfalten geschehende,
| |
| sondern durcli eine schon wiihrend der Gastrulation erfolgende Absclinurung des Keimfeldes ins Innere der Eiblasenbnlle (Oborion) †('00,
| |
| p. 203).
| |
| | |
| | |
| THE EARLY DEVELOILMENT OP THE MARSUriAl.IA, 113
| |
| | |
| | |
| vestigial representative of the shell of the presumed oviparous
| |
| common ancestor of the Metatheria and Eutheria. The
| |
| Eutherian ovum, on the other hand, has lost all trace of the
| |
| shell in correlation with its more complete adaptation to the conditions of intra-nterine development. The albumen layer is
| |
| variable in its occurrence, being present in some (e.g. rabbit)
| |
| and absent in others (e.g. pig, Assheton), whilst the zona
| |
| itself, though always present, is variable both as to its thickness and the length of time it persists.
| |
| | |
| Strangely enough, although the prevaling opinion amongst
| |
| mammalian embryologists is that the Eutherian ovum has
| |
| been derived phylogenetically from an egg of the same telolecithal and shell-bearing type as is found in the Monotremes,
| |
| no one, so far as I am aware, has ever taken the shell into
| |
| account, and ventured to consider in what way its total disappearance from an ovum already greatly reduced in size,
| |
| might affect the course of the early developmental phenomena.
| |
| That is what I propose to do here, for iu my view it is just in
| |
| the complete loss of the shell by the Eutherian ovum that we
| |
| find the key to the explanation of those remarkable differences
| |
| which are observable between the early ontogeny of the
| |
| Eutheria and Metatheria, and which culminate in the entypic
| |
| condition so distinctive of the former. The acquisition of a
| |
| shell by the Proamniota conditioned the appearance of the
| |
| amnion. The loss of the shell in the Eutheria conditioned the
| |
| occui'rence in their ontogeny of entypy.
| |
| | |
| As we have seen, the mammalian ovum, already in the
| |
| Monotremes greatly reduced iu size as compared with that of
| |
| reptiles, and quite minute in the Metatheria and Eutheria,
| |
| contains within itself neither the cubic capacity nor the food
| |
| material necessary for the production of an embryo on the
| |
| ancestral reptilian lines. We accordingly find that the
| |
| primary object of the first developmeutal processes in the
| |
| mammals has come to be the formation of a vesicle with a
| |
| complete cellular wall, capable of absorbing nutrient fluid from
| |
| the maternal uterus and of growing I'apidly, so as to provide
| |
| the space necessary for embryonal differentiation.
| |
| | |
| VOL. 56, PART 1. NEW SERIES.
| |
| | |
| | |
| 8
| |
| | |
| | |
| 114
| |
| | |
| | |
| ,T. r. HILL.
| |
| | |
| | |
| In the Monotremes this vesiculai' stage is rapidly and
| |
| directly attained as the result, firstly, of the rearrangement
| |
| of the blastomeres of the cleavage-disc to form a unilaminar
| |
| blastodermic membi'ane overlying.tbe solid yolk, and, secondly,
| |
| of the rapid extension of the peripheral (extra-embryonal)
| |
| region of the same, in contact with the inner surface of the
| |
| firm sphere furnished by the egg-envelopes. During the
| |
| completion of the blastocyst embryonal differentiation remains
| |
| in abeyance, and practically does not start until after growth
| |
| of the blastocyst is well initiated.
| |
| | |
| In the Marsupial, notwithstanding the fact that the ovum
| |
| has become secondarily holoblastic, the mode of formation
| |
| of the blastocyst is essentially that of the Monotreme.
| |
| Cleavage is of the radial type, and owing to the persistence
| |
| of the shell, wliicb with the zona forms a firm resistant
| |
| sphere enclosing the egg, the radially arranged blastomeres ai'e able to assume the form of an open ring and to
| |
| proceed directly to the formation of the unilaminar wall of
| |
| the blastocyst. The enclosing sphere provides the necessary
| |
| firm surface over which the products of division of the upper
| |
| and lower cell-rings of the 16-celled stage can respectively
| |
| spread towards opposite poles, so as to directly constitute the
| |
| formative and non-formative regions of the blastocyst wall.
| |
| In my opinion it is the persistence of the resistant shellmembrane round the ovum which conditions the occurrence
| |
| in the Marsupial of this direct method of blastocyst formation.
| |
| As in the Monotreme, so here also embryonal differentiation
| |
| commences only after the blastocyst has gi'ovvn considerably
| |
| in size.
| |
| | |
| ^ In the Eutheria, on the other hand, in the absence of the
| |
| shell-membrane, not only is the mode of formation of the
| |
| blastocyst quite different to that in the Marsupial, but
| |
| the relations of the constituent parts of the completed
| |
| structure also differ markedly from those of the homogenous parts in the latter. The cleavage process here leads
| |
| only indirectly to the formation of the blastocyst, and must be
| |
| held to be csenogeneticaily modified as compared with that of
| |
| | |
| | |
| THE EARLY DEVELOPMENT OP THE MAESUPIALIA. 115
| |
| | |
| | |
| lower mammals. In the cross-shaped arrangement of the
| |
| blastomeres in the 4-celled stage, in the occurrence of a
| |
| definite morula-stage and of the entypic condition, we have
| |
| features in which the early ontogeny of the Eutheria differs
| |
| fundamentally from that of the Metatheria. They are intimately correlated the one with the other, and are met "with in
| |
| all Eutheria, so far as known, but do not occur either in the
| |
| Prototheria or the Metatheria, so that we must regard them
| |
| as secondary features which were acquired by the primitive
| |
| Eutheria under the influence of some common causal factor
| |
| or factoi's, subsequent to their divergence from the ancestral
| |
| stock common to them and to the Metatheria. Now the crossshaped 4-celled stage and the morula-stage are undoubtedly
| |
| to be looked upon simply as cleavage adaptations of prospective
| |
| significance in regard to the entypic condition, so that the
| |
| problem reduces itself to this - how came these adaptations
| |
| to be induced in the first instance ? In view of the facts that
| |
| in the Metatheria, in the presence of the shell-membrane, the
| |
| formation of the blastocyst is the direct outcome of the cleavage
| |
| process, and is effected along the old ancestral lines without
| |
| any enclosure of the formative cells by the non-formative,
| |
| whilst in the Eutheria, in the absence of the shell-membrane, blastocyst formation results only indirectly from the
| |
| cleavage-process, is effected in a way quite different from
| |
| that characteristic of the Metatheria, and involves the
| |
| complete enclosure of the formative by the non-formative
| |
| cells, I venture to suggest that the cleavage adaptations
| |
| which I'esult in the entypic condition were acquired in the first
| |
| instance as the direct outcome of the total loss by the already
| |
| greatly reduced Eutlierian ovum of the shell-membrane.^
| |
| This view necessarily implies that the presence of a thick
| |
| zona such as occurs round the ovum in certain Eutheria is
| |
| secondary, and what we know of this membrane in existing
| |
| Eutheria is at all events not adverse to that conclusion.
| |
| | |
| This suggestion I first put foi'ward in a course of lectures on the
| |
| early ontogeny and placentation of the Mammalia delivered at the
| |
| University of Sydney in 1904.
| |
| | |
| | |
| 116
| |
| | |
| | |
| J. P. HILL.
| |
| | |
| | |
| Amongst tlie Marsupials the zona is quite thin (about -00] 6
| |
| imn. in Dasyurus), presumptive evidence that it was also thin
| |
| in the ancestral stock from which the Meta- and Eutheria
| |
| diverged, whilst amongst the Eutheria themselves the zona,
| |
| as Robinson ('03) has pointed out, is not only of very varying
| |
| thickness, but persists round the ovum for a very varying
| |
| period iu different species. It appears to be thinnest in the
| |
| mouse ('001 mm.), in most Eutheria it is considerably thicker
| |
| (•01 mm., bat, dog, rabbit, deer), whilst in Cavia it reaches
| |
| a thickness of as much as -02 mm. In those forms in which
| |
| the blastocyst early becomes embedded in, or attached to, the
| |
| mucosa, the zona naturally disappears early. In the rat,
| |
| mouse and guinea-pig it disappears before the blastocyst is
| |
| formed. Hubrecht failed to find it in the 2-celled egg of
| |
| Tupaia, and it was already absent in the 4-celled stage of
| |
| Macacus nemestrinus, discovered by Selenka and described by Hubrecht. On the other hand, it may persist for
| |
| a much longer period, up to the time of appearance of the
| |
| primitive streak (rabbit, dog, ferret). These facts sufficiently demonstrate the variability of the zona in the Eutherian
| |
| series, and its early disappearance in certain forms before the
| |
| completion of the blastocyst stage shows that it can have no
| |
| supporting function in i-egard to that.
| |
| | |
| Postulating, then, the disappearance of the shell-membrane
| |
| and the presence of a relatively thin, non-resistant zona (with
| |
| perhaps a layer of albumen) round the minute yolk-poor ovum
| |
| of the primitive Eutherian, and remembering that the ovum
| |
| starts with certain inherited tendencies, the most immediate
| |
| and pressing of which is to produce a blastocyst comprising
| |
| two differentiated groups of cells, the problem is how, in the
| |
| absence of the old supporting sphere constituted by the eggenvelopes, can such a vesicular stage be most easily and
| |
| most expeditiously attained ? The Eutherian solution as we see
| |
| it in operation to-day is really a very simple one, and withal a
| |
| noteworthy instance of adaptation in cleavage (Lillie, '99).
| |
| In the absence of any firm supporting membrane round the
| |
| egg, and the consequent impossibility of the blastomeres pro
| |
| | |
| THE EARLV DEVELOPMENT OF THE MARSUPIALIA. 117
| |
| | |
| ceecling- at once to forna the blastocyst wall, they are under
| |
| the necessity of keeping together, and to this end cleavage
| |
| has become adapted. For the ancestral radial arrangement
| |
| of the blastomeres in the 4-celled stage, characteristic of the
| |
| Monotreme and Marsupial, there has been substituted a
| |
| cross-shaped grouping into two pairs, and, as the outcome of
| |
| this adaptive alteration in the cleavage planes, there results
| |
| from the subsequent divisions, not an open cell-ring, as in tbe
| |
| Marsupial, but a compact cell-group or morula. In this we
| |
| again encounter precisely the same differentiation of the
| |
| blastomeres into two categories, respectively formative
| |
| (embryonal) and non-formative (trophoblastic) insignificance,
| |
| as is found in the 16-celled stage of the Marsupial, but, since
| |
| the two groups of cells are here massed together, and in the
| |
| absence of any firm enclosing sphere, cannot spread independently so as to form directly the wall of the blastocyst,
| |
| there has arisen the necessity for yet other adaptive modifications. Attention has already been directed to the tardiness
| |
| of differentiation in the embryonal region of the Monotreme
| |
| and Marsupial blastocyst, and here in the minute Eutherian
| |
| morula we find what is, perhaps, to be looked upon as a
| |
| further adaptive exaggeration of this same feature in the
| |
| inertness which is at tirst displayed by the formative cells,
| |
| and which is in marked contrast with the activity shown by
| |
| the non-formative ectodermal cells.^ It is these latter, it
| |
| | |
| * The inertness of the formative cell-mass is accounted for by Assheton
| |
| ('98, p. 251) as follows : “ Now, as the epiblast plays the more prominent
| |
| part in the formation of the l^nlk of the embi-yo dui-ing the earliest
| |
| stages, it clearly would be useless for tlie embryonic part to exhibit
| |
| much energy of growth until the old conditions [in particular sufficient
| |
| room for embryonal differentiation] were to a certain extent regained ;
| |
| hence the lethargy exhibited by the embryonic epiblast in mammals
| |
| during the first week of develoxunent. No feature of the early stages of
| |
| the mammalian embryo is more striking than this inertness of the
| |
| embryonic eiriblast - or, as I should nowjrrefer to call it, simply epiblast
| |
| - during the first few days.†Assheton, it should be remembered, holds
| |
| that the inner cell-mass of Eutheria furnishes only the embryonal
| |
| ectoderm.
| |
| | |
| | |
| 118
| |
| | |
| | |
| J. P. HILL.
| |
| | |
| | |
| should be recollected, which exhibit the greatest growthenergy during the formation of the blastocyst in the Monotreme and Marsupial, and so their greater activity in the
| |
| Eutherian tnoi'ula is only what might be expected. Dividing
| |
| more rapidly than the formative cells, they gradually grow
| |
| round the latter, and eventually form a complete outer layer
| |
| enveloping the inert formative cell-group. This process oFovergrowth or epiboly is entirely comparable in its effect with the
| |
| spreading of the extra-embryonal region of the unilamiiiar
| |
| blastodermic membrane in the Monotreme to enclose the yolkmass, and with that of the non-formative cells in the Marsupial
| |
| to complete the lower hemisphere of the blastocyst, growlh
| |
| round an inert central cell-mass being here substituted for
| |
| growth over the inner surface of a I'esistant sphere constituted
| |
| by the egg-envelopes, such as occurs during the formation of
| |
| the blastocyst in the Monotreme and Marsupial. .Just as the
| |
| first objective of the cleavage process in the latter is to effect
| |
| the completion of the cellular wall of the blastocyst, so hei*e
| |
| the same objective recurs, and is attained in the simplest
| |
| possible way in the new circumstances, viz. by the I'apid envelopment of the formative by the, non-formative cells. Thus
| |
| at the end of the cleavage process in the EutheiJan we have
| |
| formed a solid entypic morula in which an inner mass of
| |
| formative cells is completely surrounded by an outer enveloping layer of non-formative or ti'opho-ectodermal cells, homogenous with the extra-embryonal ectoderm of the Sauropsidan
| |
| and Monotreme and the non-formative region of the unilaminar blastocyst of the Marsupial. Conversion of the solid
| |
| morula into a hollow blastocyst capable of imbibing fluid
| |
| from the uterus and of growing rapidly now follows. Intraor intercellular vacuoles appear below the inner cell-mass, by
| |
| the confluence of which the blastocyst cavity is established,
| |
| and the inner cell-mass becomes separated from the enveloping layer of tropho-ectoderm, except over a small area where
| |
| the two remain in contact.
| |
| | |
| The complete enclosure of the formative cells of the inner
| |
| cell-mass by the non-formative ectodermal cells of the
| |
| | |
| | |
| THE EARLY DEVELOPMENT OF THE MARSUPIALIA. 119
| |
| | |
| | |
| enveloping layer which produces this peculiar entypic condition in the Eutherian blastocyst, I would interpret, then, as
| |
| a purely adaptive phenomenon, which in the given circumstances effects in the simplest possible way the early completion
| |
| of the blastocyst wall, and whose origin is to be traced to
| |
| that reduction in size and in its envelopes which the Eutherian
| |
| ovum has suffered in the course of phylogeny, in adaptation
| |
| to the conditions of intra-uterine development. In particular,
| |
| starting with a shell-bearing ovum, already minute and
| |
| undergoing its development in utero, I see in the loss of
| |
| the shell such as has occurred in the Eutheria an intelligible
| |
| explanation of the first origin of those adaptations which
| |
| culminate in the condition of entypy. I am therefore wholly
| |
| unable to accept the view of Hubrecht (^08, p. 78), that " what
| |
| Selenka has designated by the name of Entypie is - from
| |
| our point of view - no secondary phenomenon, but one
| |
| which repeats very primitive featui*es of separation between
| |
| embryonic ectoderm and larval envelope in invertebrate
| |
| ancestors.â€
| |
| | |
| I see no reason for supposing that the intimate relationship
| |
| which is early established in many Eutheria between the
| |
| trophoblastic ectoderm and the uterine mucosa has had anything to do with the origination of the entypic condition. In
| |
| ray view such intimate relationship involving the complete
| |
| enclosui'e of the blastocyst in the mucosa only came to be
| |
| established secondarily, after entypy had become the rule.
| |
| On the other hand, the peculiar modifications of the entypic
| |
| condition met with in rodents with “^inversion†(e.g. i-at,
| |
| mouse, guinea-pig) are undoubtedly to be correlated, as Van
| |
| Beneden also believed ('99, p. 332), with the remarkably early
| |
| and complete enclosure or implantation of the germ in the
| |
| mucosa such as occurs in these and other Eutheria. Similar
| |
| views are expressed by Selenka in one of his last contributions
| |
| to mammalian embryology. He writes ('00, p. 205) - “Dass
| |
| die Entypie des Keimfeldes und die Blattinversion begiinstigt
| |
| wil'd durch die friihzeitige Yerwachsung der Eiblase mit dem
| |
| Uterus, ist nicht in Abrede zu stellen. Aber da dieser
| |
| | |
| | |
| 120
| |
| | |
| | |
| J. P, HILL.
| |
| | |
| | |
| Prozess auch in solclieu Eiblasen dei- Saugetiere vorkommen
| |
| kanii, die iiberhaupt nichb, odei- erst spiiter mifc dem Uterus
| |
| verwachsen, so kaiiu die Keimfeld-Entypie zwar durch die
| |
| frube Verwacbsung veraiilasst, aber nicht ausscldiesslich
| |
| liervorgerufeii werclen.†He goes on to remark that - “Die
| |
| Vorbedingimgeti zur Eutypie miissen in der Struktur der verwachseuden Eiblase gesucht werden/^ and expi-esses his
| |
| agreement with the views of Van Beneden as to tlie significance to be attributed to the early cleaviige phenomena in
| |
| Eutheria.
| |
| | |
| The attitude of the illustrious Belgian embryologist whose
| |
| loss ws have so recently to deplore, towards this problem is
| |
| clearly set forth in the last memoir which issued from his
| |
| hand. “Je suis de ceux,^' he wrote (T9, p. 332), “qui pensent
| |
| que toute Pembryologie des Mammiferes placentaires temoigue
| |
| quTls derivent d'animaux qui, comme les Sauropsides et les
| |
| Mouotremes, produisaieut des oeufs meroblastiques. Je ne
| |
| puis a aucun point de vue me rallier aux idees contraires formulees eb defendues par Hubrecht. L^hypothese de Hubrecht
| |
| se heurte a des difiicultes morpliologiques et physiologiques
| |
| insurmontables : elle laisse inexpliquee Pexistence, chez les
| |
| Mammiferes placentaires, d'une vesicule ombilicale et dTne
| |
| foule de caracteres commnns a tons les Amniotes et distiuctifs
| |
| de ces auimaux.'^ Holding this view of tlie origin of the
| |
| Eutheria, Van Beneden based his interpretation of their early
| |
| ontogenetic phenomena on the belief that “ la reduction progressive du volume de Poeuf d'une part, le fait de son
| |
| developpement iutrauterin de hautre ont dii avoir une influence preponderante sur les premiers processus evolutifs.â€
| |
| | |
| Balfour, in his classical treatise, had already some eighteen
| |
| years earlier expressed precisely the same view. “The
| |
| features of the development of the placental Mammalia,^' he
| |
| wrote (‘Mem. Edn.,^ vol. iii, p. 289), “receive their most
| |
| satisfactory explanation on the hypothesis that their ancestors
| |
| were provided with a large-yolked ovum like that of Sauropsida. The food-yolk must be supposed to have ceased to be
| |
| developed on the establishment of a maternal nutrition through
| |
| | |
| | |
| THE EAELY DEVELOPMENT UE THE MAKSUPJALIA. 121
| |
| | |
| | |
| the uterus. . . . The embryonic evidence of the common
| |
| | |
| origin of Mammalia and Sauropsida, both as concerns the
| |
| formation of the layers and of the embryonic membranes is
| |
| as clear as it can be.'''
| |
| | |
| That view of tlie derivation of the Mammalia receives, I
| |
| venture to think, striking confirmation from the observations
| |
| and conclusions set forth in the preceding pages of this
| |
| memoir, and from it as a basis all attempts at a phylogenetic
| |
| interpretation of the early ontogenetic phenomena in the
| |
| Mammalia must, I am convinced, take their origin. Such an
| |
| attempt I have essayed in the foregoing pages, with what
| |
| success the reader must judge.
| |
| | |
| | |
| Addendum.
| |
| | |
| The memoir of Prof. 0. Van der Stricht, entitled “La structure de I'cBuf des Mammiferes (Chauve-souris, Vesperugo
| |
| noctula) : Troisieme Partie†(‘Mem. de PAcad. roy. de
| |
| Belgique,' 2nd ser., t. ii, 1909), came into my hands only
| |
| after my own paper had readied its final form, and therefore
| |
| too late for notice in the body of the text. In this extremely
| |
| valuable contribution, Van der Stricht gives a detailed
| |
| account of the growth, maturation, fertilisation, and early
| |
| cleavage-stages of the ovum of Vesperugo, illustrated by a
| |
| superb series of drawings and photo-micrographs. All I can
| |
| do here, however, is to direct attention to that section of the
| |
| paper entitled “ Phenomeues de deutoplasmolyse an pole
| |
| vegetatif de I'ceuf†(pp. 92 - 96), in which the author describes
| |
| the occurrence in the bat's ovum of just such a process of
| |
| elimination of surplus deutoplasmic material as I have
| |
| recorded for Dasyurus. Van der Stricht's interpretation of
| |
| this phenomenon agrees, I am glad to find, with my own.
| |
| He writes (pp. 92-93): “ Ce deutoplasme rudimentaire, i\
| |
| peine ebauche dans I'ovule des Mammiferes, parait etre
| |
| encore trop abundant dans I'oeuf de Chauve-souris, car ces
| |
| materiaux de reserve, en partie inutiles, sont partiellement
| |
| elimines, expulses de la cellule.â€
| |
| | |
| | |
| 122
| |
| | |
| | |
| .T. P. HILL.
| |
| | |
| To this pi'ocess of elimination of surplus deutoplasm he
| |
| applies the name deutoplasmolyse,†and states that Ce
| |
| phenomene consiste dans I'apparition de lobules vitellins
| |
| multiples, en nombre tres variable, a la surface du vitellus au
| |
| niveau du pole vegetatif. Ces bourgeons a peu pres tous de
| |
| meme grandeur, les uns etant cependant un peu plus volumineux que les autres, apparaissent dans le voisinage des globules
| |
| polaires et presentent la structure du deutophisme. 11s sont
| |
| formes de vacuoles claires, a I'interieur desquelles on aper^oit
| |
| parfois de petits grains vitellins, dont il a ete question plus
| |
| haut. . . . Ce processus de deutoplasmolyse devient
| |
| | |
| manifeste surtout apres I'expulsion du second globule polaire,
| |
| pendant la periode de la fecondation. 11 pent etre tres
| |
| accentue, au stade du premier fuseau de segmentation et au
| |
| debut de la segmentation de I'oeuf, notamment sur des ovules
| |
| divises en deux et en quatre (figs. 59, 61, 62, d).†It would
| |
| therefore appear that, whilst in Dasyurus the surplus deutoplasm is eliminated always prior to the completion of the
| |
| first cleavage and in the form of a single relatively large
| |
| spherical mass, in Vesperugo it is cast off generally, though
| |
| not invariably, before cleavage begins, and in the form of a
| |
| number of small separate lobules.
| |
| | |
| | |
| List op References.
| |
| | |
| '94. Assheton, R. - “ A Re-investigation into the Early Stages of the
| |
| Development of the Rabbit,†‘ Quart. Journ. Micr. Sci.,' vol. 34.
| |
| | |
| '98. “ The Development of the Pig during the Pirst Ten Days,â€
| |
| | |
| ‘ Quart. Journ. Micr. Sci.,' vol. 41.
| |
| | |
| '98. “ The Segmentation of the Ovum of the Sheep, with Obser
| |
| vations on the Hypothesis of a Hypoblastic Origin for the
| |
| Trophoblast,†‘ Quart. Journ. Micr. Sci.,' vol. 41.
| |
| | |
| '08. “ The Blastocyst of Capra, with Some Remarks upon the
| |
| | |
| Homologies of the Germinal Layers of Mammals,†‘Guy's
| |
| Hospital Reports,' vol. Ixii.
| |
| | |
| '09. “Professor Hubrecht's Paper on the Early Ontogenetic
| |
| | |
| Phenomena in Mammals ; An Appreciation and Criticism,â€
| |
| ‘ Quart. Journ. Micr. Sci.,' vol. 54.
| |
| | |
| | |
| 123
| |
| | |
| | |
| THE EAELY DEVELOPMENT OF THE MARSUPIALIA.
| |
| | |
| '97. Bonnet. R. - “ Beitriige zur Embvyologie des Himdes,†‘ Anatomische Hefte,' Bd. ix.
| |
| | |
| '01. “ Erste Fortsetzimg,†‘ Anatomisclie Hefte,' Bd. xvi.
| |
| | |
| '87. Caldwell, W. H. - “ The Erabiyology of Monotremata and Marsnpialia,†Part I, ‘ Phil. Trans. Roy. Soc.,' vol. clxxviii B.
| |
| | |
| '95. Duval, M. - “Etudes sur I'embryologie des Oliciropteres,†‘ Joura.
| |
| de I'Anat. et de la Pliysiol.,' t. xxxi.
| |
| | |
| '86. Heape, W. - “ The Development of the Mole (Talpa Europea), the
| |
| Ovarian Ovum, and Segmentation of the Ovum,†‘Quart. Joum.
| |
| Micr. Sci.,' vol. 26.
| |
| | |
| '97. Hill, J. P. - “ The Placentation of Perameles,†‘ Quart. Journ.
| |
| Micr. Sci.,' vol. 40.
| |
| | |
| '00. “ On the Foetal Membranes, Placentation and Parturition of
| |
| | |
| theNative Cat(Dasyurus viverrinus),†‘Anat. Anz.,'Bd.xviii.
| |
| | |
| '88. Hubrecht, A. A. W. - “ Keimbliitterbildung und Placentation des
| |
| Igels,†‘ Anat. Anz.,' Bd. iii.
| |
| | |
| '89. “ Studies in Mammalian Embryology : (1) The Placentation
| |
| | |
| of Erinaceus europaeus, with Remarks on the Physiology of
| |
| the Placenta,†‘ Quart. Joura. Micr. Sci.,' vol. 30.
| |
| | |
| '95. “ Die Phylogenese des Amnions und die Bedeutung des
| |
| | |
| Trophoblastes,†‘ Verhand. Kon. Akad. v. Wetensch. Amsterdam,'
| |
| vol. iv.
| |
| | |
| '02. “ Fiirchung und Keimblattbildung bei Tarsius Spectrum,â€
| |
| | |
| ‘ Yerhand. Kon. Akad. v. Wetensch. Amsterdam,' vol. viii.
| |
| | |
| '04. “ The Ti'ophoblast,†‘ Anat. Anz.,' Bd. xxv.
| |
| | |
| '08. “ Early Ontogenetic Phenomena in Mammals, and their
| |
| | |
| Bearing on oim Intei'pretation of the Phylogeny of the Vertebrates,†‘ Quart. Joura. Micr. Sci.,' vol. 53. .
| |
| | |
| '09. “The Foetal Membranes of the Vertebrates,†‘ Proc.
| |
| | |
| Seventh Interaational Congress, Boston Meeting,' August 19th
| |
| to 24th, 1907.
| |
| | |
| '00. Jenkinson, J. W. - “A Re-investigation of the Early Stages of the
| |
| Development of the Mouse,†‘ Quart. Journ. Micr. Sci.,' vol. 43.
| |
| | |
| '06. “ Remarks on the Germinal Layers of Vertebrates and on
| |
| | |
| the Significance of Germinal Layers in General,†‘ Mem. and
| |
| Proc. Manchester Lit. and Philos. Soc.,' vol. 1.
| |
| | |
| '01. Keibel, F. - “Die Gastrulation und die Keimblattbildung der
| |
| Wirbeltiere,†‘ Ergebnisse der Anatomie und Entwickelungsgeschichte ' (Merkel u. Bonnet), Bd. x.
| |
| | |
| “ Die Entwickelung der Rehes bis zui* Anlage des Meso
| |
| blast,†‘ Arch, fiir Anat. u. Physiol. Anat. Abth.'
| |
| | |
| | |
| ' 02 .
| |
| | |
| | |
| 124
| |
| | |
| | |
| J. r. Hii,L.
| |
| | |
| | |
| 0/. Lams, H., and Doonne, J. - “ Nouv^elles recheivhes sur la Maturation et la Fecondation de I'cenf des Maminiferes,†‘ Arch de Biol.,'
| |
| t. xxiii.
| |
| | |
| 03. Lee, T. Gr. ‘Implantation of the Ovum in Sf)ermoi)hilus
| |
| tridecemlineatus, Mitcli.,†‘ Mark Anniv. Vol.,' Art. 21.
| |
| | |
| '99. Lillie, F. R. - ‘ Adaptation in Cleavage,†‘Biol. Lect. Wood's
| |
| Holl.,' 1897 - 98 (Ginn & Co., Boston).
| |
| | |
| '09. MacBride, E. W. - “ The Formation of the Layers in Amphioxus
| |
| and its bearing on the Interjiretation of the Eai'ly Ontogenetic
| |
| Processes in other Vertebrates,†‘ Quart. Journ. Micr. Sci.,' vol. 54.
| |
| | |
| 03. Robinson, A. Lectures on the Early Stages in the Development
| |
| of Mammalian Ova and on the Formation of the Placenta in
| |
| Different Groups of Mammals,†‘ Journ. of Anat. and Physiol.,'
| |
| vol. xxxviii.
| |
| | |
| 86. Selenka, E. ‘ Studien iiber Entwickelungsgeschichte der Thiere,'
| |
| IV (1 and 2), “ Das Opossum (Didelphys virginianaj,†Wiesbaden.
| |
| | |
| '91. ‘‘ Beutelfuchs und Kiinguruhratte ; zur Entsteliungs
| |
| geschichte der Amnion der Kantjil (Tragulus javanicus) ;
| |
| Att'en Ost-Indiens,†‘ Studien fiber Entw. der Tiere,' H. 5, Erste
| |
| Hiilfte.
| |
| | |
| '00. ‘ Studien hber Entw. der Tiere,' H. 8, Menschenaffen.
| |
| | |
| “ III, Entwickelung des Gibbon (Hylobates und Sianianga),â€
| |
| Wiesbaden : 0. W. Kreidel.
| |
| | |
| '94. Semon, R. - “Zur Entwickelungsgeschichte der Monotremen,â€
| |
| ‘ Zool. Forschungsreisen iin Australien, etc.,' Bd. ii. Lief 1.
| |
| | |
| '95. Sobotta, J. “ Die Befruchtung und Furchung des Eies der Mans,â€
| |
| ‘ Arch, fiir Mikr. Anat.,' Bd. xlv.
| |
| | |
| '75. Van Beneden, E. - †La Maturation de I'cEuf, la fecondation et les
| |
| Iiremieres phases du develoiipement embryonnaire des Mammiferes d'apres les recherches faites sur Je Lapin,†‘ Bull, de I'Acad.
| |
| roy. des sciences, des lettres, et des beauxaits de Belgique,' t. xl.
| |
| | |
| '80 “ Recherches sur I'emliryologie des Maminiferes, la forma
| |
| tion des feuillets chez le Lapin,†‘ Arch, de Biologie,' t. i.
| |
| | |
| '99 “ Recherches sur les premiers Stades du developpement du
| |
| | |
| Murin (Vespertilio murinus),†‘Anat. Anz.,' Bd. xvi.
| |
| | |
| '03 Van der Stricht, O. - ‘‘La Structure et la Polarite de I'ceuf de
| |
| Chauve-Souris (V. noctula),†‘ Comptes rendus de I'Association
| |
| des Anatomistes, V“ Session, Liege.'
| |
| | |
| “ La Structure de I'ceuf des Maminiferes. Premiere partie,
| |
| | |
| L'oocyte an stade de I'accroissement,†“Arch, de Biologic,'
| |
| t. xxi.
| |
| | |
| | |
| '04
| |
| | |
| | |
| THE EARLY DEVELOEMENT OF THE MARSTJriALIA. 125
| |
| | |
| | |
| '05 Van cler Stvidit, O. - “ La Stvuctnre de I'ceuf des MammifOTes.
| |
| Denxieine partie, Structure de I'ceuf ovarique de la femme,†‘ Bull,
| |
| de I'Acad. Roy. de Medicine de Belgique,' Seance du 24 J uin, 1905.
| |
| | |
| '97 Wilson, J. T., and Hill, J. P. - “ Observations upon the Development and Succession ot the Teeth in Perameles; togethei with
| |
| a Contribution to the Discussion of the Homologies of the Teeth
| |
| in Marsupial Animals,†‘ Quart. Journ. Micr. Sci., vol. xxxix.
| |
| | |
| '03 “ Primitive Knot and Early Gastrnlation Cavity co
| |
| existing with independent Primitive Streak in Ornithorhynchus,â€
| |
| ‘ Proc. Roy. Soc.,' vol. Ixxi.
| |
| | |
| '07 “ Observations on the Development of Ornithorhyn
| |
| chus,†‘ Phil. Trans. Roy. Soc.,' Series B, vol cxcix.
| |
| | |
| | |
| EXPLANATION OF PLATES 1-9,
| |
| | |
| Illustrating Prof. J. P. Hill's paper on “ The Early Development of the Marsupialia, with Special Reference to the
| |
| Native Cat (Dasyurus vi verrinus).â€
| |
| | |
| [All figures are from specimens of Dasyurus, unless otherwise indicated. Drawings were executed with the aid of Zeiss's camera lucida,
| |
| except figs. 61-63, which were drawn from photographs.]
| |
| | |
| List of Common Reference Letters.
| |
| | |
| Ab7i. Abnormal blastomei'e, fig. 37. alh. Albumen, eg. Coagulum.
| |
| d. p. Discus proligerus. d. z. Deutoplasmic zone. emb. a. Embryonal
| |
| area. emb. ect. Embiyonal ectoderm, ent. Entoderm. /. ep. Follicular
| |
| epithelium. /. a. Formative area of blastocyst wall. /. c. Formative
| |
| cell. /. z. Formative zone. i. c. Internal cell, fig. 34. Z. eat. Limit of
| |
| extension of entoderm. Z. p. Incomjilete ai'ea of blastocyst wall at lower
| |
| pole. p. b'. First polar body. p. b'. s. First polar spindle, p. V. s.
| |
| Second polar spindle, p. s. Perivitelline space, s. m. Shell-membrane.
| |
| sp. Sperm in albumen. Zr. ect. Non-formative or trophoblastic ectoderm (tropho-ectoderm). y.b. Yolk-body. z. p. Zona.
| |
| | |
| PLATE 1.
| |
| | |
| Fig. 1. - Photo-micrograph (x 150 diameters) of the full-grown
| |
| ovarian ovum, '27 X ‘26 mm. diameter. The central deutoplasmic
| |
| zone (cZ. z.) and the peripheral formative zone (/. z.), in which the vesicular nucleus ('QS X '03 mni. diameter) is situated, are clearly distinguishable. The zona (z. p.) measures •0021-'0025 mm. in thickness.
| |
| Outside it are the follicular epithelial cells of the discus proligerus
| |
| (d.p.), which is thickened on the upper side of the figure, where it
| |
| becomes continuous with the membrana granulosa. (D. v i v., 21 . vii .
| |
| '04, Hermann's fluid and iron-hsematoxylin.)
| |
| | |
| Fig. 2. - Photo-micrograph ( X. 150) of ripe ovarian ovum (in which
| |
| first polar body is separated and second polar spindle is present, though
| |
| neither is visible in figure), '29 X '23 mm. maximum diametei'. FoUicle
| |
| 1'4 X IT mm. diameter. The ovum exhibits an obvious polarity.
| |
| Deutoplasmic zone {d. z.) in upper hemisphere ; formative zone (/. z.)
| |
| foi-ming lower. (D. v i v., 14, 26 . vii . '02, Flemming's fluid and
| |
| iron-haematoxylin.)
| |
| | |
| Fig. 3. - Photo-microgi'aph ( x 150) of ripe ovarian ovum ('28 x '24
| |
| mm. diameter) with first polar body (p. bK) and second polar spindle.
| |
| First polar body, •026-‘03 x '01 mm. Second polar spindle, '013 mm.
| |
| in length. (D. v i v., 14, 26 . vii . '02, Flemming's fli;id and ironhaematoxylin.)
| |
| | |
| Fig. 4. - Photo-micrograph (x 256) of ovarian ovum in process of
| |
| growth (“pseudo-alveolar†stage). Ovum, ‘26 X '20 mm. diameter.
| |
| Zona, •0017-‘002 mm. in thickness. (D. v i v., 14, 26 . vii . '02,
| |
| Hermann, iron-haematoxylin.)
| |
| | |
| Fig. 5. - Photo-microgi-aph (X 1250) of peripheral i-egion of ripe
| |
| ovarian ovum ('28 X T26 mm. diameter) with first polar spindle ('015
| |
| X '013 mm.). (D. v i v., 23 . vii . '02, Ohlmaicher's fluid, iron-haema
| |
| toxylin.)
| |
| | |
| Fig. 6. - Photo-micrograph (x 1250) of peripheral region of ripe
| |
| ovarian ovum ('26 X T8 mm.), showing first polar body (p. b'.) ('03 X
| |
| •006 mm.). (D. v i v., 14, 26 . vii . '02, Flemming, iron-hfematoxylin.)
| |
| | |
| Fig. 7. - Photomicrograph ( X 1250) of periplieral region of ovum, fig.
| |
| 3, showing portion of first polar body (p. 5'.), and the second polar
| |
| spindle. The dark body lying between p. 5'. and the surface of the
| |
| ovum is a displaced red blood-corpuscle.
| |
| | |
| Figs. 8 and 9. - Photo-micrographs ( X about 84) of unsegmented ova,
| |
| respectively '33 mm. and '35 mm. in diameter, from the uterus, taken
| |
| immediately after their transference to the fixing fluid (picro-nitroosmic acid), showing the shell-membrane (s. m.), laminated albumen
| |
| {alb.), with sperms (sp.), the zona (z. p.), perivitelline space {p. s.), and
| |
| the body of the ovum, with its formative (/. z.), and deutoplasmic {d. z.)
| |
| zones. (D. v i v., 15, 19 . vii . '01.)
| |
| | |
| Fig. 10. - Photo-micrograph ( X 150) of section of imsegmented ovum
| |
| almost immediately after its passage into the uterus, showing the very thin sliell-inembvane externally (s. m.) (about '0016 mm. in thickness),
| |
| the albumen {alb.), zona (z-i?.), and the deutoplasmic {d. z.) and formative
| |
| (/. z.) zones of its cytoplasmic body. The male pronucleus is visible in
| |
| the formative zone. Diameter of entire egg about '29 mm. (D. viv.,
| |
| 15, 19 . vii . '01, Picro-nitro-osmic and iron-hffimatoxylin.)
| |
| | |
| Fig. 11. - Photo-micrograph ( X 150) of section of unsegmented ovum
| |
| from the uterus, slightly older than that of fig. 10. Diameter of entire
| |
| egg in fresh state •34-'35 mm., of the ovum proper '3 X ‘28 mm. ; thickness of shell, -0024 mm. In the figure the female pronucleus is visible
| |
| near the centre of the formative zone (/. z.), and the male pronucleus
| |
| lies a little above it and to the right. The perivitelline space (jJ.s.)
| |
| is pai-tiaUy occupied by coagulum. (D . viv., 21 . v . '03, f. Hermann,
| |
| iron-hsematoxylin.)
| |
| | |
| PLATE 2.
| |
| | |
| Fig. 12. - Photo-micrograph ( X 150) of an unsegmented ovum from
| |
| the irterus, of the same batch as that of fig. 11, and '34 mm. in diameter.
| |
| The two pronuclei are visible in the central region of the formative
| |
| zone.
| |
| | |
| Fig. 13. - Photo-microgi-aph ( X 330) of uterine ovum. Stage of first
| |
| cleavage spindle. Diameter, '315 mm. (D. viv., 1, 15 . vii . '01, f.
| |
| Picro-nitro-osmic, iron-hiematoxylin.)
| |
| | |
| Fig. 14. - Photo-micrograph ( X about 78) of egg in the 2-celled stage,
| |
| taken immediately after its transference to the fixing fluid. Lateral
| |
| view. y. b. Yolk body. Diameter of entire egg about "34 mm. (D . viv.,
| |
| 1, 15 . vii . '01. Picro-nitro-osmic.)
| |
| | |
| Fig. 15. - Photo-microgi'aph (x about 78) of another 2-celled egg,
| |
| seen from lower pole. Diameter, '35 mm. (D. viv., 4 B, 23 . vi . '02.
| |
| Perenyi's fluid.)
| |
| | |
| Fig. 16. - Photo-micrograph (x about 78) of another 2-celled egg,
| |
| of the same batch as preceding. End view, showing one of the two
| |
| blastomeres and the yolk -body (y. b.).
| |
| | |
| Fig. 17. - Photo-micrograph (x 150) of vertical section of 2-celled
| |
| egg, "34 mm. in diameter, showing the shell-membrane ('0064 mm. thick),
| |
| traces only of the albumen, the zona (z.p.), and the two blastomeres (the
| |
| left one measuring, from the sections, T6 x T8 x TO mm., its nucleus
| |
| ‘031 X ‘027 mm. ; the right one, T6 x T9 X "09 mm., its nucleus, '03 x
| |
| •028 mm.). Note the differentiation in their cytoplasmic bodies.
| |
| (D . viv., 6, 21 . vii . '01, Picro-nitro -osmic and iron-hsematoxylin.)
| |
| | |
| Fig. 18. - Photo-micrograph (x 150) of vertical section of 2-celled
| |
| egg, '32 mm. in diameter, with shell-membrane '005 mm. thick, showing
| |
| the two blastomeres, and enclosed between their upper ends the yolk body {y. b.). (D . viv., 1, 15 . vii . '01, f. Picro-nitro-osmic, iron-htematoxylin.)
| |
| | |
| Figs. 19 and 20. - Photo-micrographs ( x about 70) of 4-eelled eggs
| |
| taken immediately after transference to Perenyi's fluid. Fig. 19, side
| |
| view, showing yolk-body (y. h.) ; fig. 20, polar view. Diameter of entire
| |
| egg about -35 mm. (D . viv., 14 b, 18 . vi . '02. Perenyi.)
| |
| | |
| Fig. 21. - Photo- micrograph (x about 70) of another 4-celled egg,
| |
| from the same batch as the preceding, seen from lower pole.
| |
| | |
| Fig. 22. - Photo-micrograph (x 150) of section of 4-ceUed egg of
| |
| same batch as those of figs. 19 and 20. The two right and the two
| |
| left blastomeres respectively form pairs, so that the plane of the first
| |
| cleavage is parallel with the sides of tlie plate, that of the second with
| |
| the top and bottom of the same. The two left blastomeres are still
| |
| connected by a narrow cytoplasmic bridge. Thickness of shell,
| |
| •0072 mm.
| |
| | |
| Fig. 23. - Photo-micrograph ( x 150) of a vertical section through
| |
| a 4-celled egg. ‘35 mm. in diameter, showing two of the blastomeres
| |
| and a small portion of the yolk-body {y. b.). Note, as in fig. 22, the
| |
| marked diflierentiation in the cytoplasm of the blastomeres. (D. viv.,
| |
| 4, 27 . vi . '01. Picro-nitro-osmic, iron-hsematoxylin.)
| |
| | |
| Figs. 24 and 25. - Photo-micrographs ( x 140) of horizontal sections
| |
| through a 16-celled egg, '38 mm. diameter, fig. 24 showing the eight
| |
| larger, more yolk-rich cells of the lower (non-formative) ring, and fig. 25
| |
| the eight smaller, less yolk-rich cells of the upper (formative) ring.
| |
| Shell ‘0075 mm. in thickness, yolk-body (not included in the figures)
| |
| 'll X TO mm. in diameter. (D. viv., 3 b, 26 . vi . '01; 15, f and |.
| |
| Picro-nitro-osmic and iron-hsematoxylin.)
| |
| | |
| Fig. 26. - Photo-micrograph (x 140) of a vertical section of an egg
| |
| of the same batch and size as that represented in figs. 24 and 25, but
| |
| with seventeen cells - formative = 9 (6 + [1 X 2] + 1) in division ;
| |
| non-formative = 8. Two of the formative cells (/. c.) of the upper ring
| |
| are seen enclosing between them the faintly mai'ked yolk-body {y. b.),
| |
| and below them two of the much more opaque non-formative cells
| |
| {tr. ect.) of the lower ring.
| |
| | |
| | |
| PLATE 3.
| |
| | |
| Fig. 27. - Photo-micrograph (x about 76) of the just completed
| |
| blastocyst, '39 mm. in diameter. From a spirit specimen. The dark
| |
| spherical mass (eg.) in the blastocyst cavity is simply coagulum, produced by the action of the fixative (picro-nitro-osmic acid) on the
| |
| albuminous fluid which fills the blastocyst cavity. (D. viv., 2 b,
| |
| 16 . vii . '01.)
| |
| | |
| | |
| Fig. 28. - Plioto-anicrogi-apli ( X about 76) of a blastocyst of the same
| |
| batch as the preceding, •45 mm. in diameter. From a spirit specimen.
| |
| eg. Coagulum.
| |
| | |
| Fig. 29. - Photo-micrograph (x about 75) of another blastocyst,
| |
| •45 mm. diameter, of the same batch as the preceding, but taken
| |
| immediately after transference to the fixative. Viewed from the upper
| |
| pole. y. b. Tolk-body seen through the unilaminar wall.
| |
| | |
| Fig. 30. - Photo-micrograph ( X about 75) of a blastocyst of the same
| |
| batch as the preceding, about '39 mm. in diameter, in which the cellular
| |
| wall has not yet been completed over the lower polar region.
| |
| | |
| Fig. 31. - Photo-micrograph ( X 140) of a section of a blastocyst,
| |
| •39 mm. diameter, of the same batch as the preceding and at precisely
| |
| the same developmental stage, the cellular wall having yet to be completed over the lower polar region (l.p.). In the blastocyst cavity is
| |
| seen the yolk-body (y. b.) partially surroixnded by a mass of coagulum
| |
| (eg.). (D. viv., 2 B, 16 . vii . '01, m. = '39, Picro-nitro-osmic and
| |
| iron-hsematoxylin.)
| |
| | |
| Fig. 32. - Photo-micrograph ( X 140) of another blastocyst, ^41 mm.
| |
| in diameter, of the same batch as the preceding, also 'with the cellular
| |
| wall still absent over the lower polar region. Shell-membrane ‘0075 mm.
| |
| in thickness, y. b. Tolk-body. c. g. Coagulum. The cellular wall
| |
| comprises about 130 cells.
| |
| | |
| Fig. 33. - Photo-micrograph ( X 140) of a blastocyst of the same batch
| |
| as the preceding, with a complete unilaminar cellular wall. y. b. Yolkbody, in contact with inner surface of wall, in the region of the upper
| |
| pole.
| |
| | |
| Fig. 34. - Photo-micrograph (x 100) of a section of a blastocyst
| |
| •57 mm. in diameter, i. c. Internal ceU. (D . vi v., 29 . vi . '04, y . Pici^onitro-osmic.)
| |
| | |
| Fig. 35. - Photo-micrograph (x 100) of a section of a blastocyst, '73
| |
| mm. diameter, of the same batch as the pi^eceding, shell, ^0045 mm.
| |
| thick.
| |
| | |
| Fig. 36. - Photo-micrograph (x 100) of a section of a blastocyst -66
| |
| mm. diameter, of the same batch as the pi-eceding. Lower hemisphere
| |
| opposite yolk-body {y. b.) formed of larger cells than upper. Hermann
| |
| fixation.
| |
| | |
| Fig. 37. - Photo-micrograph (x 140) of section of an abnormal
| |
| vesicle, 397 mm. diameter of the same batch as the normal vesicles
| |
| represented in figs. 27-33. abn. large binucleate cell, regarded as a
| |
| blastomere of the lower hemisphex^e which has failed to divide in noi^mal
| |
| fashion, cf . text, p. 42.
| |
| | |
| PLATE 4.
| |
| | |
| Fig. 38 - Photo-micrograpli ( x 10) of entire blastocyst 4'5 mm. diameter to show the junctional line {j. 1.) between formative and nonformative regions. From a spirit specimen. (D . viv., /3, 25 . vii . '01.
| |
| Picro-nitro-osmic.)
| |
| | |
| Fig. 39. - Photo-microgi-aph ( x about 10) of an entire blastocyst,
| |
| 4'5 mm. diameter with distinct embryonal area {emh. a.). (D. viv., 5,
| |
| 18 . vii . '01.)
| |
| | |
| Fig. 40. - Photo-micrograph { X 10) of entire blastocyst about 5 mm.
| |
| diameter showing embryonal area' {emh. a.), peripheral limit of entoderm (1. ent.), and the still unilaminar region of the wall {tr. ect.). (D.
| |
| viv., 8 . vi . '01.)
| |
| | |
| Fig. 41. - Photo-micrograph ( x 150) of an in toto preparation of the
| |
| wall of a blastocyst of 3'5 mm. diameter. (D . viv., 16, 21 . vii . '01.)
| |
| | |
| Fig. 42. - Photo-micrograph (x 150) of an in toto preparation of the
| |
| wall of a blastocyst of 3'25 mm. diameter, j. 1. Junctional line between
| |
| the formative (/. a.) and non-formative {tr. ect.) regions of the wall.
| |
| (D. viv., 24 . vii . '01.)
| |
| | |
| Figs. 43 and 44. - Photo-micrographs (x 150) of in toto preparations
| |
| of the wall of 4'5 mm. blastocyst showing the jimctional line between
| |
| the formative (/. a.) and non-formative {tr. ect.) regions. (D. viv.,
| |
| P, 25 . vii . '01. Picro-nitro-osmic and Ehrlich's hsematoxylin )
| |
| | |
| Fig. 45. - Photo-micrograph ( x 150) of a corresponding preparation
| |
| of the wall of a more advanced 4'5 mm. blastocyst ('99 stage), in which
| |
| the two regions of the wall are now clearly distinguishable. (D. viv.,
| |
| 8.7. '99. Picro-nitro-osmic, Ehrlich's hsematoxylin.)
| |
| | |
| Fig. 46. - Photo -micrograph ( x 150) of a corresponding preparation
| |
| of a slightly more advanced blastocyst ('04 stage). (D. viv., 6 . 7 . '04.
| |
| Picro-nitro-osmic, Ehrlich's hsematoxylin.)
| |
| | |
| PLATE 5.
| |
| | |
| Fig. 47. - Photo-micrograph (x 150) of an in toto preparation of the
| |
| formative region of a 6 . 7 . '04 blastocyst, showing the proliferation
| |
| of spherical interaal cells refeiTed to in the text, p. 53.
| |
| | |
| Fig. 48. - Photo-micrograph ( X 150) of an in toto preparation of the
| |
| wall of a vesicle of the same batch as that represented in fig. 39, in
| |
| which a small part of the junctional line between the embryonal ectodenn and the extra-embryonal {tr. ect.) is visible, the free edge of the
| |
| entoderm {ent.) not having reached it. (D. viv., 5, 18 . vii . '01. Picronitro-osmic, Ehrlich's hsematoxylin.)
| |
| | |
| | |
| Fig. 49. - Photo-micrograpli ( X 150) of a con-esponding preparation
| |
| of a vesicle of the same batch as the preceding, in which the wavy and
| |
| irregularly thickened free edge of the entoderm {ent.) practically
| |
| coincides with the junctional line and so conceals it from view.
| |
| | |
| Fig. 50. - Photo-micrograph (x 150) of an in to to preparation of a
| |
| vesicle (8 . vi . '01 batch) viewed from the inner surface as in the corresponding preceding figures. The entoderm in the region of the embryonal
| |
| ax-ea has been removed, so that one sees the inner surface of the embryonal
| |
| ectoderm [emh. ect.) ; it is still in situ, though not in a quite intact condition over the adjoining portion of extra-embryonal ectoderm. The
| |
| entoderm has not yet extended over the region indicated by the reference
| |
| line to tr. ect., so that here the extra-embryonal ectoderm is cleai-ly
| |
| visible. The jimctional line is apparent. (D. viv., 8 . vi . '01. Picronitro-osmic. Ehrlich's hsematoxylin.)
| |
| | |
| Fig. 51 (Plate 3). - Photo-microgi-aph ( X 310) of a section of a 30celled egg of Perameles obesula; egg b, '24 X '23 mm. diameter,
| |
| showing the xinilaniinar layer formed by the blastomeres.
| |
| | |
| Fig. 52 (Plate 3). - Photo-micrograph (x 240) of a section of a
| |
| blastocyst of P. nasuta '29 X •26 mm. diameter, showing the shellmembrane {s.vi.), zona (z.p.), and the unilaminar celhxlar waU. The
| |
| portion of the latter adjacent to the reference lines is composed of
| |
| smaller but thicker cells than the remainder.
| |
| | |
| PLATE 6.
| |
| | |
| Figs. 53 and 54. - Drawings ( X 84) of a 6-celled egg '34 mm. diameter,
| |
| fig. 53 showing a side view and fig. 54 a view from the lower pole.
| |
| Observe the characteristic I'ing-shaped arrangement of the blastomeres.
| |
| y. b. Yolk -body, the shell-membrane, albumen layer with sperms included, and the zona are readily distinguishable. Outlines drawn with
| |
| the aid of the camera lucida immediately after transference of the egg
| |
| to the fixing fluid. (D . viv., 22, 16 . vii . '01.)
| |
| | |
| Figs. 55 and 56. - Drawings ( X about 88) of a 16-ceUed egg (about ‘37
| |
| mm. diameter) as seen fx'om the side and lower pole respectively, from
| |
| the same batch as the eggs represented in figs. 24, 25, and 26. The charactei'istic aii'angement of the blastomex'es in two sxxpex'imposed, open
| |
| x'ings (each of eight cells) and the diffex'ence in size between the cells of
| |
| the two riixgs are evident. The ix'x-egxxlar body (c.g.) seen ixx the cleavage
| |
| cavity in fig. 56 is a mass of coagxxluixx. Dx'aunx from a spix'it specimen.
| |
| The albumen layer as represented in fig. 56 is too thick. (D. viv.,
| |
| 3 B, 26 . vi . '01.)
| |
| | |
| Figs. 57 and 58. - Drawings (x about 85) of a 12-celled egg (-38 xixm.
| |
| diameter) as seen from the side axxd lower pole respectively. Four of the blastomeres of the 8-ceHed stage have already divided (4 + 4x2)
| |
| = 12. From a spirit specimen and from same batch as preceding.
| |
| | |
| Fig. 59. - Drawing ( x about 88) of a 31-celled egg ('375 mm. diameter)
| |
| as seen from the lower pole. From a spirit specimen and fi-om the same
| |
| batch as the preceding. The irregular body in the blastocyst cavity is
| |
| formed by coagulnm. Formative cells = 16; non- formative = 14 + 1
| |
| in division.
| |
| | |
| Fig. 60. - Drawing ( X about 88) of another 31-celled egg ('375 diameter)
| |
| from the same batch as the preceding. Side view.
| |
| | |
| Fig. 61. - Drawing (x 100) of an entire blastocyst (‘39 mm. diameter)
| |
| from the same batch as those shown in figs. 27-29.
| |
| | |
| Fig. 62. - Drawing ( x about 80) of an entire blastocyst (‘4 mm.
| |
| diameter) from the same batch as the preceding.
| |
| | |
| Fig. 63. - Drawing (x 80 of an entire blastocyst ('6 mm. diameter)
| |
| made from a photogi'aph taken directly after transference of the specimen to the fixing fluid. Cells of lower hemisphere with imich more
| |
| marked perinuclear areas of dense cytoplasm than those of the upper.
| |
| D. viv., 2, 11 . vii . '01.)
| |
| | |
| Fig. 64. - Section of the wall of a blastocyst, 2'4 mm. diameter
| |
| (x 630). (D. viv., 7 . vi . '01.)
| |
| | |
| Figs. 65, 66, 67. - Drawings (x 630) of small portions of in toto
| |
| preparations of the formative region of 6 . 7 . '04 blastocysts to demonstrate the mode of origin of the primitive entodermal cells {ent., fig. 67).
| |
| Fig. 65 shows a dividing entodermal mother-cell in position in the
| |
| unilaminar wall, siuTounded by larger lighter staining cells (prospective
| |
| embryonal ectodermal cells). In fig. 66 is seen a corresponding cell, a
| |
| poi-tion of whose cell-body has extended inwards so as to underlie
| |
| (overlie in figure) one of the ectodermal cells of the wall. . In fig. 67
| |
| are seen two entodermal cells, evidently sister-cells, the products of the
| |
| division of such a cell as is seen in figs. 65 or 66. One of them (the
| |
| upper) is still a constituent of the unilaminar wall, the other {ent.) is a
| |
| primitive entodermal cell, definitely internal. (D . viv ., 6 . 7 . '04. Picronitro-osmic, Ehrlich's haematoxylin.)
| |
| | |
| | |
| PLATE 7.
| |
| | |
| Figs. 68, 69, 70. - Drawings (x 630) of portions of preparations
| |
| similar to the above. For description see text. (D. viv., 6, 7, '04.)
| |
| | |
| Fig; 71. - Drawing (x about 630) of a portion of an in toto preparation of the formative region of an '01 blastocyst showing two
| |
| primitive entodermal cells, one of them in division. (D. viv., (3,
| |
| 25 . vii . '01. Picro-nitro-osmic and Ehrlich.)
| |
| | |
| | |
| | |
| Fig. 72. - D rawing (x 630) corresponding to the above, from the
| |
| formative region of a 6 . 7 . '04 blastocyst, also showing two primitive
| |
| entodermal cells, evidently sister-cells.
| |
| | |
| | |
| PLATE 8.
| |
| | |
| Figs. 73, 74, 76. - Sections of the formative region of 6.7. '04 blastocysts, showing the attenuated shell-membrane, the unilaminar waU, and
| |
| in close contact with the inner surface of the latter, the primitive entodermal cells {ent.) ( X 630).
| |
| | |
| Fig. 75. - Section corresponding to the above, showing an entodermal
| |
| mother-cell {ent.), part of whose cell-body nndei'lies the adjacent ectodermal cell of the wall. The spheroidal inwardly projecting cell on the
| |
| left is probably also an entodermal mother-cell (x 630).
| |
| | |
| Fig. 77. - Section ( x 630) of the non-formative I'egion of a 6 . 7 . '04
| |
| blastocyst.
| |
| | |
| Fig. 78. - Section ( X 630) of the embryonal ai'ea, and the adjoining
| |
| portion of the still imilaminar extra-embryonal region of a blastocyst of
| |
| the 5 . '01 stage, emb. ect. Embryonal ectoderm, ent. Entoderm, tr.
| |
| ect. Extra-embryonal ectoderm (tropho-ectoderm). The position of the
| |
| junctional line is readily recognisable. (D . vi v. , 5, 18 . vii . '01. Picronitro-osmic and Delafield's hsematoxylin.)
| |
| | |
| Fig. 79. - Section (x 630) through the corresponding regions in an
| |
| 8 . vi . '01 blastocyst. Note the thickening of the embryonal ectoderm
| |
| {emb. ect.), and the peripheral extension of the entoderm {ent.) below
| |
| the tropho-ectoderm. (D. viv., 8 . vi . '01. Picro-nitro-osmic and
| |
| Lelafield.)
| |
| | |
| Fig. 80. - Section (x 600) through the formative (embryonal) region
| |
| of a blastocyst of P. nasuta, 1‘3 mm. in diameter. It is thicker than
| |
| that of the Dasyure blastocyst at the corresponding stage of development ; the primitive entodermal cells are well mai-ked.
| |
| | |
| Fig. 81. - Section ( x 600) corresponding to the above from another
| |
| 1-3 mm. blastocyst of P. nasuta, of the same batch as the preceding,
| |
| but apparently very slightly earlier, the entodermal cells being stiU in
| |
| process of separating from the unilaminar wall. ent. Entoderm, tr. ect.
| |
| Tropho-ectoderm.
| |
| | |
| | |
| PLATE 9.
| |
| | |
| Fig. 82. Section (x about 430) of a section of a blastocyst of M.
| |
| ruficollis -35 mm. in diameter, showing the major portion of the
| |
| formative region (/. a.) and a small portion of the non-formative {tr. ect.).
| |
| | |
| | |
| | |
| The shell-membrane varies in thickness in the sections from (J05 min.
| |
| over the former region to '003 mm. over the latter.
| |
| | |
| Figs. 83, 84, 85. - Drawings ( X 630) of small portions of the formative
| |
| (and in fig. 83 of the adjoining portion of the non-formative) region of
| |
| the above blastocyst of M. ruficollis more highly magnified, ent.
| |
| Primitive entodermal cells. Note in fig. 83 a cell of the wall in division,
| |
| the axis of the spindle being oblique to the surface.
| |
| | |
| | |
| | |
| | |
| J. P. Hill, Photo.
| |
| | |
| Watbslow & Sows LiMlTiiD, Collotype.
| |